In highly polarized cells like neurons, cytoskeleton dynamics play a crucial role in establishing neuronal connections during development and are required for adult plasticity. Actin turnover is particularly important for neurite growth, axon path finding, branching and synaptogenesis. Motoneurons establish several thousand branches that innervate neuromuscular synapses (NMJs). Axonal branching and terminal arborization are fundamental events during the establishment of synapses in motor endplates. Branching process is triggered by the assembly of actin filaments along the axon shaft giving rise to filopodia formation. The unique contribution of the three actin isoforms, α-, β- and γ-actin, in filopodia stability and dynamics during this process is not well characterized. Here, we performed high resolution in situ hybridization and qRT-PCR and showed that in primary mouse motoneurons α-, β- and γ-actin isoforms are expressed and their transcripts are translocated into axons. Using FRAP experiments, we showed that transcripts for α-, β- and γ-actin become locally translated in axonal growth cones and translation hot spots of the axonal branch points. Using live cell imaging, we showed that shRNA depletion of α-actin reduces dynamics of axonal filopodia which correlates with reduced number of collateral branches and impairs axon elongation. Depletion of β-actin correlates with reduced dynamics of growth cone filopoida, disturbs axon elongation and impairs presynaptic differentiation. Also, depletion of γ-actin impairs axonal growth and decreases axonal filopodia dynamics. These findings implicate that actin isoforms accomplish unique functions during development of motor axons. Depletions of β- and γ-actin lead to compensatory upregulation of other two isoforms. Consistent with this, total actin levels remain unaltered and F-actin polymerization capacity is preserved. After the knockdown of either α- or γ-actin, the levels of β-actin increase in the G-actin pool indicating that polymerization and stability of β-actin filaments depend on α- or γ-actin. This study provides evidence both for unique and overlapping function of actin isoforms in motoneuron growth and differentiation. In the soma of developing motoneurons, actin isoforms act redundantly and thus could compensate for each other’s loss. In the axon, α-, β- and γ-actin accomplish specific functions, i.e. β-actin regulates axon elongation and plasticity and α- and γ-actin regulate axonal branching. Furthermore, we show that both axonal transport and local translation of α-, β- and γ-actin isoforms are impaired in Smn knockout motoneurons, indicating a role for Smn protein in RNA granule assembly and local translation of these actin isoforms in primary mouse motoneurons. / In stark polaren Zellen wie den Neuronen ist die Etablierung neuronaler Netzwerke ein entscheidender Faktor bei der Entwicklung des zentralen Nervensystems und spielt für die adulte Plastizität eine wesentliche Rolle. Besonders die Aktindynamik ist wichtig für das Neuritenwachstum, die axonale Wegfindung und Verzweigung, sowie die Synaptogenese. Motoneurone bilden mehrere tausend terminale Verzweigungen aus, um neuromuskuläre Endplatten (NMJ) zu innervieren. Die axonale Verzweigung ist ein fundamentales Ereignis bei Ausbildung synaptischer Verbindungen zwischen Motoneuron und innerviertem Muskel. Die Axonverzweigung geschieht durch die Polymerisierung von Aktin entlang des Axonschafts, was zur Entstehung von Filopodien und Lamellopodien führt. Allerdings ist die genaue Funktion der drei Aktin-Isoformen (α-, β- and γ-Actin), im Zusammenhang mit der Regulation der Filopodienstabilität und deren Dynamik, noch weitestgehend unbekannt. Somit konnten wir in dieser Arbeit mit Hilfe hoch sensitiver in situ Hybridisierungs- und qRT PCR Techniken zeigen, dass in primären Mausmotoneuronen alle drei Aktinisoformen (α-, β- und γ) exprimiert, und deren Transkripte entlang des axonalen Kompartiments transportiert werden. Unsere FRAP Daten weisen darauf hin, dass α-, β- und γ-Aktin sowohl im Wachstumskegel als auch an sogenannten „Translation Hot Spots“ innerhalb axonaler Verzweigungspunkte lokal synthetisiert werden. Anhand von „Live Cell Imaging“ Experimenten konnten wir dann zeigen, dass ein α-Aktin Knockdown die Dynamik axonaler Filopodien stark reduziert, und als Folge, die Anzahl von axonalen Verzweigungen und die Axonlänge verringert ist. Hingegen geht ein β-Aktin Knockdown mit reduzierter Filopodiendynamik im Wachstumskegel und betroffener Differenzierung präsynaptischer Strukturen einher. Veränderungen des axonalen Wachstum und der Filopodiendynamik sind ebenfalls bei einem γ-Aktin Knockdown zu beobachten. Diese Daten weisen darauf hin, dass die drei Aktinisoformen unterschiedliche Funktionen bei der Entwicklung von Motoraxonen haben. Darüber hinaus zeigen unsere Daten, dass die Herunterregulation einer Aktinisoform durch eine erhöhte Expression der beiden anderen Isoformen kompensiert wird. Dieser Kompensationsmechanismus erlaubt es, die gesamte Aktinmenge und somit die F-Aktin-Polymerisation in der Zelle aufrechtzuerhalten. Sehr interessant dabei ist die Beobachtung, dass nach einem α- oder γ-Actin Knockdown das G/F-Verhältnis verändert ist, so dass die Menge an β-Aktin im G-Aktin Pool steigt und im F-Aktin Pool abnimmt. Daher beruhen Polymerisation und Stabilität von β-Aktin auf den α-, und γ-Aktinisoformen. Zusammenfassend lässt sich sagen, dass alle drei Aktinisoformen übergreifende Funktionen während Wachstum und Differenzierung von Motoneuronen haben. Im Zellkörper von sich entwickelnden Motoneuronen übernehmen sie ähnliche Aufgaben und können sich somit gegenseitig kompensieren. Im Gegensatz dazu sind die Funktionen im axonalen Kompartiment wesentlich spezifischer. Hier reguliert β-Aktin axonales Wachstum und Plastizität, während α- und γ-Aktin eine entscheidende Rolle bei der Ausbildung axonaler Verzweigungen haben. Unsere Arbeit lässt nun Rückschlüsse über mögliche Funktionen des SMN Proteins beim Aufbau der sogenannten „RNA Granules“ und lokaler Proteinbiosynthese der verschiedenen Aktinisoformen in primären Mausmotoneuronen zu.

Die retrovirale Replikation in der eukaryotischen Zelle erfordert den Export Intron-enthaltender Transkripte aus dem Kern ins Cytoplasma. Bei HIV-1 wird dieser nucleocytoplasmatische Transport durch den viralen Transaktivator Rev vermittelt. Rev ist ein Shuttle-Protein, das sowohl ein Kernimportsignal (NLS) als auch ein Leucin-reiches Kernexportsignal (NES) besitzt. Nach der Bindung von Rev an eine spezifische RNA Sekundärstruktur, das sogenannte Rev Response Element (RRE) interagieren zelluläre Faktoren mit dem NES von Rev, wodurch der Kernexport vermittelt wird. Neben dem generellen Exportrezeptor CRM1 konnte auch der eukaryotische Initiationsfaktore 5A (eIF-5A) als ein Bindungspartner von Rev identifiziert werden. In dieser Arbeit konnte nun gezeigt werden, daß eIF-5A ein essentieller Faktor für den Rev-vermittelten RNA Export ist. Mikroinjektionen von eIF-5A-Antikörpern und der eIF-5A-M14 Mutante in Kerne von Xenopus Oocyten, sowie Bindungsstudien in Lösung haben gezeigt, daß eIF-5A als ein Adapterprotein fungiert, das upstream des generellen Exportrezeptors CRM1 wirkt. eIF-5A bindet dabei an das Rev-NES und vermittelt dadurch eine effiziente Bindung dieses NES an CRM1, wodurch der effiziente Export des Rev/RNA-Komplexes stattfinden kann. Da die zelluläre Funktion von eIF-5A noch unbekannt war, wurden Overlay Blot Assays auf Xenopus Oocytenkernhüllen durchgeführt, um Kernproteine zu finden, die mit eIF-5A interagieren. Dies führte zur Identifikation des Transkriptionsfaktors IIIA als einen Bindungspartner von eIF-5A. TFIIIA ist ein Exportfaktor für die Oocyten-Typ 5S rRNA in Amphibien Oocyten und besitzt wie Rev ein Leucin-reiches NES. Aufgrund einer Analyse dieses RNA Exportweges konnte nun gezeigt werden, daß eIF-5A auch in diesem spezifischen Exportweg als Adapter wirkt, der das NES des TFIIIA mit dem Exportrezeptor CRM1 verbindet und dadurch den Export des TFIIIA/5S rRNA-Komplexes vermittelt. Eine weitere zelluläre Funktion von eIF-5A konnte beim Export der CD83 mRNA in Dendritischen Zellen gefunden werden. Es konnte gezeigt werden, daß der Export der CD83 mRNA durch das RNA-bindende Protein HuR und durch den generellen Exportrezeptor CRM1 vermittelt wird. Durch den HuR Lignaden APRIL, der ein Rev-ähnliches, Leucin-reiches NES besitzt, wird dabei die Bindung an CRM1 vermittelt. Des weiteren konnte gezeigt werden, daß eIF-5A an diesem RNA Export beteiligt ist. Wie auch beim Rev-vermittelten RRE RNA Export und dem TFIIIA-vermittelten 5S rRNA Export wirkt eIF-5A als ein Adapter, der das NES des HuR-Liganden APRIL mit CRM1 verbindet, wodurch der Export des CD83 mRNA/HuR/APRIL Komplexes stattfinden kann. Neben TFIIIA und verschiedenen Nucleoporinen, konnte Kernaktin als ein weiterer Bindungspartner von eIF-5A identifiziert werden. In dieser Arbeit durchgeführte Mikroinjektionsexperimente mit Antikörpern gegen Aktin sowie verschiedenen Aktin-bindende Drogen konnten zeigen, daß Kernaktin scheinbar generell in Exportprozesse involviert ist. Mit Hilfe verschiedener Aktin-bindender Proteine (Latrunculin B und Swinholide A) konnte gezeigt werden, daß eine lösliche oder oligomere Form, nicht jedoch Aktinfilamente, funktionell an Kernexportprozessen beteiligt sind. Durch die Analyse Kernaktin-bindender Proteine konnten bereits die beiden Nucleoporine CAN/Nup214 und p62, die beide an Exportprozessen beteiligt sind, als Bindungspartner identifiziert werden. Außerdem ergaben sich höchst interessante Hinweise auf die Beteiligung eines, bis jetzt noch nicht identifizierten, Kernproteins auf eine Beteiligung am Aktin-vermittelten Kernexport.

Abstract not available

Cells contain an elaborate cytoskeleton which plays a major role in a variety of cellular functions including: maintenance of cell shape and dimension, providing mechanical strength, cell motility, cytokinesis during mitosis and meiosis and intracellular transport. The cell cytoskeleton is made up of three types of protein filaments: the microtubules, the intermediate filaments and the actin cytoskeleton. These components interact with each other to allow the cell to function correctly. When functioning incorrectly, disruptions to many cellular pathway have been observed with mutations in various cytoskeletal proteins causing an assortment of human disease phenotypes. Characterization of these filament systems in different cell types is essential to the understanding of basic cellular processes and disease causation. The studies in this thesis are concerned with examining specific cytoskeletal tropomyosin-defined actin filament systems in skeletal muscle. The diversity of the actin filament system relies, in part, on the family of actin binding proteins, the tropomyosins (Tms). There are in excess of forty Tm isoforms found in mammals which are derived from four genes: α, β, γ and δTm. The role of the musclespecific Tms in striated muscle is well understood, with sarcomeric Tm isoforms functioning as part of the thin filament where it regulates actin-myosin interactions and hence muscle contraction. However, relatively little known about the roles of the many cytoskeletal Tm isoforms. Cytoskeletal Tms have been shown to compartmentalise to form functionally distinct filaments in a range of cell types including neurons (Bryce et al., 2003), fibroblasts (Percival et al., 2000) and epithelial cells (Dalby-Payne et al., 2003). Recently it has been shown that cytoskeletal Tm, Tm5NM1 defines a cytoskeletal structure in skeletal muscle called the Z-line associated cytoskeleton (Z-LAC) (Kee et al., 2004).The disruption of this structure by over-expression of an exogenous Tm in transgenic mice results in a muscular dystrophy phenotype, indicating that the Z-LAC plays an important role in maintenance of muscle structure (Kee et al., 2004). In this study, specific cytoskeletal Tms are further investigated in the context of skeletal muscle. Here, we examine the expression, localisation and potential function of cytoskeletal Tm isoforms, focussing on Tm4 (derived from the δ- gene) and Tm5NM1 (derived from the γ-gene). By western blotting and immuno-staining mouse skeletal muscle, we show that cytoskeletal Tms are expressed in a range of muscles and define separate populations of filaments. These filaments are found in association with a number of muscle structures including the myotendinous junction, neuromuscular junction, the sarcolemma, the t-tubules and the sarcoplasmic reticulum. Of particular interest, Tm4 and Tm5NM1 define cytoskeletal elements in association with the saroplasmic reticulum and T-tubules, respectively, with a separation of less than 90 nm between distinct filamentous populations. The segregation of Tm isoforms indicates a role for Tms in the specification of actin filament function at these cellular regions. Examination of muscle during development, regeneration and disease revealed that Tm4 defines a novel cytoskeletal filament system that is orientated perpendicular to the sarcomeric apparatus. Tm4 is up-regulated in both muscular dystrophy and nemaline myopathy and also during induced regeneration and focal repair in mouse muscle. Transition of the Tm4-defined filaments from a predominsnatly longitudinal to a predominantly Z-LAC orientation is observed during the course of muscle regeneration. This study shows that Tm4 is a marker of regeneration and repair, in response to disease, injury and stress in skeletal muscle. Analysis of Tm5NM1 over-expressing (Tm5/52) and null (9d89) mice revealed that compensation between Tm genes does not occur in skeletal muscle. We found that the levels of cytoskeletal Tms derived from the δ-gene are not altered to compensate for the loss or gain of Tm5NM1 and that the localisation of Tm4 is unchanged in skeletal muscle of these mice. Also, excess Tm5NM1 is sorted correctly, localising to the ZLAC. This data correlates with evidence from previous investigations which indicates that Tm isoforms are not redundant and are functionally distinct (Gunning et al., 2005). Transgenic and null mice have also allowed the further elucidation of cytoskeletal Tm function in skeletal muscle. Analyses of these mice suggest a role for Tm5NM1 in glucose regulation in both skeletal muscle and adipose tissue. Tm5NM1 is found to colocalise with members of the glucose transport p fibres and analysis of both transgenic and null mice has shown an alteration to glucose uptake in adipose tissue. Taken together these data indicate that Tm5NM1 may play a role in the translocation of the glucose transport molecule GLUT4. In addition to this Tm5NM1 may play a role in adipose tissue regulation, since over-expressing mice found to have increased white adipose tissue and an up-regulation of a transcriptional regulator of fat-cell formation, PPAR-γ. / Doctor of Philosophy (PhD)

<p>Lipopolysaccharide (LPS) is a structure of the gram-negative bacteria that protect from chemicals and works as a stabilization component for the membrane. Studies show that LPS also may have a function to avoid immune defense. In this project we investigate two <em>Salmonella enterica</em> variants with different LPS conformation. The wild-type Salmonella got an originally LPS structure and the mutant form had a defect one. The bacteria were transfected with a green fluorescent protein (GFP) to allow measuring of phagocytosis. Monocytes were isolated from human blood and were incubated for several days with cytokines to give dendritic cells. The cells were exposed to each type of <em>Salmonella</em> and incubated for different times. After labeling with phalloidin and studies with fluorescent microscopy, phagocytosis and F-actin were measured. The results show that it is a difference in phagocytosis and F-actin depending on LPS conformation. That means that LPS may have a decisive role for the pathogenicity of <em>Salmonella</em>.</p>

Le cytosquelette dactine est une structure dynamique impliquée dans de nombreux processus biologiques tels que les mouvements cellulaires, la phagocytose ou encore la mitose. En plus de son intervention dans ces différents événements essentiels pour lhoméostasie de la cellule, de nombreuses études ont démontré quil était également capable dinfluer sur des voies de transduction notamment en modulant lactivité de protéines kinases ou de facteurs de transcription. Un facteur de transcription important est le facteur de transcription NF-κB. Ce facteur de transcription joue un rôle majeur dans la régulation de nombreux processus cellulaires tels que les réponses immunitaires innée et adaptative, la réponse inflammatoire, lapoptose et la division cellulaire. Il peut être activé en réponse à de nombreux stimuli tels que les cytokines pro-inflammatoires, les produits bactériens ou viraux ou encore suite à un stress oxydant. Malgré les différentes études démontrant que le cytosquelette dactine est capable de moduler certaines voies de signalisation et que certains stimuli capables dactiver le NF-κB, comme le LPS et le TNFα, sont également associés à des modifications du cytosquelette dactine, peu de travaux ont été réalisés afin de déterminer limpact des perturbations du cytosquelette dactine sur lactivation de cet important facteur de transcription. Ces différents arguments nous ont donc poussé à étudier le rôle des perturbations du cytosquelette dactine dans les voies dactivation du NF-κB. Ainsi, dans une première partie, nous avons étudié leffet de plusieurs agents perturbant le cytosquelette dactine [la Cytochalasine D (CytD), le Jasplakinolide (JP) et la Latrunculine B (Lat B)] sur lactivation du NF-κB. Nous avons pu mettre en évidence que ces différentes substances sont capables dinduire la voie classique dactivation du NF-κB uniquement dans des cellules myélomonocytaires. De plus, nous avons également observé que ces agents sont capables dinduire la production despèces réactives à loxygène (ROS) Dans un second temps, nous nous sommes intéressés à leffet de la CytD sur lactivation du NF-κB dans des cellules myélomonocytaires induites par le TNFα ou le LPS. Nous avons pu démontrer que la CytD promouvoit la voie classique du NF-κB dans les cellules induites par le LPS. En effet, il semblerait que la CytD agisse notamment sur cette voie en augmentant le temps de résidence du récepteur à cet inducteur, le TLR4, à la surface de la membrane plasmique. Parallèlement à ces observations, nous avons pu mettre en évidence que la CytD augmente la phosphorylation de certains résidus de la sous-unité RelA du NF-κB induite par les deux inducteurs classiques ce qui permet un meilleur recrutement de la RNA polymérase II sur le promoteur endogène de la chémokine IL-8.

Integrins are a family of conserved cell surface receptors found throughout the animal kingdom. They comprise 24 dimers in mammals, and regulate a number of processes including cell survival, differentiation, and migration. These complex cellular responses involve processes such as cell attachment, spreading, and various signaling pathways, which in turn depend on the composition of the extracellular environment, on its mechanical properties, and involved integrin types. This thesis focuses on identifying molecules that signal downstream of integrins and how integrin-induced signals may differ dependent on the type of mechanical stimulus that is given. In Paper I, we show that cell spreading and the activation of AKT is regulated by the catalytic PI3K isoform p110α. An intact β1 integrin cytoplasmic tail and actin polymerization was needed for spreading, whereas the presence of FAK or SRC, or the interaction between p110α and RAS was dispensable. Paper II reports that the RICTOR-mTOR complex (TORC2) acts as the kinase downstream of β1 integrins in order to phosphorylate AKT on Ser473, which was functionally linked to cell survival. β1 integrins activated both AKT1 and AKT2, but seemed to prefer AKT2. The investigation of several receptor types with regard to their requirement of TORC2, PAK, and ILK for AKT Ser473 phosphorylation revealed that different kinds of receptors engage specific enzyme combinations depending on cell type and context. In the third paper, we demonstrate that adhesion- and mechanical stretch-induced integrin signaling lead to divergent protein phosphorylation patterns, and that most signals from cell adhesion were not dependent on intracellular contractility. This indicates that integrin ligand binding and mechanical stretch induce signaling via distinct mechanisms. Reactive oxygen species (ROS) derived from different cellular sources modulated these responses. Stretching primarily induced phosphorylation of ERK1/2, and this signal was markedly increased by a derivative of the antioxidant ascorbate and extracellularly administered catalase. The robust AKT phosphorylation in response to adhesion was almost completely abolished with an inhibitor targeting mitochondrial ROS, whereas phosphorylation levels were only marginally affected in stretch assays. Similar results were obtained with siRNA knock-down of a critical subunit of ROS-producing NADPH oxidases.

Cell motility is a fundamental process, enabling cells to migrate, for instance during embryogenesis, tissue repair and defense. Force is generated by two protein systems, which also participate in cell proliferation, control macromolecular and organelle distribution and determine the fine structure of the cell interior. The major components of these are actin and tubulin, respectively, and they are referred to as the microfilament and the microtubule systems. This thesis focuses on tropomyosin, one of many microfilament associated proteins coupled to actin dynamics and organization and expressed in several isoform variants. Altered distribution and isoform expression of tropomyosin are signatures of malignant cells and are dealt with in the current thesis. The presence of tropomyosin isoforms in protruding lamellipodia of migrating cells is demonstrated, and a method to fractionate tropomyosin depending on its organization in an easily extractable, and a more tightly bound cytoplasmic form is presented. Analysis of the loosely associated tropomyosin fraction by gel filtration chromatography revealed that most of the tropomyosins in this fraction exist in a multimeric form. It was also observed that the distribution of tropomyosin varied between non-transformed and transformed cells with most of the isoforms enriched in the loosely bound fraction in the latter category of cells. Possibly this reflects the extensive reorganization of the microfilament system observed in cancer cells and which, depending on the context, can be normalized by introduction of certain tropomyosin isoforms. Many anti-cancer drugs target the microtubule system, inhibit cell division and promote apoptosis. Here it is shown that picropodophyllin, which has promising anticancer properties has a destabilizing effect on microtubules and via the microfilament system causes cells to detach from their substratum. Furthermore, picropodophyllin interferes with stimulation of the insulin-like growth factor receptor, which is involved in growth stimulation, differentiation and survival and whose expression is up-regulated in cancer cells.

Lipopolysaccharide (LPS) is a structure of the gram-negative bacteria that protect from chemicals and works as a stabilization component for the membrane. Studies show that LPS also may have a function to avoid immune defense. In this project we investigate two Salmonella enterica variants with different LPS conformation. The wild-type Salmonella got an originally LPS structure and the mutant form had a defect one. The bacteria were transfected with a green fluorescent protein (GFP) to allow measuring of phagocytosis. Monocytes were isolated from human blood and were incubated for several days with cytokines to give dendritic cells. The cells were exposed to each type of Salmonella and incubated for different times. After labeling with phalloidin and studies with fluorescent microscopy, phagocytosis and F-actin were measured. The results show that it is a difference in phagocytosis and F-actin depending on LPS conformation. That means that LPS may have a decisive role for the pathogenicity of Salmonella.

Rho GTPases are critical regulators of actin cytoskeletal dynamics. The three most well characterized Rho GTPases, Rac1, RhoA and Cdc42 share a common inhibitor, RhoGDI. It is only recently becoming clear how upstream signals cause the selective release of individual Rho GTPases from RhoGDI. For example, our laboratory showed that diacylglycerol kinase zeta (DGKz), which converts diacylglycerol (DAG) to phosphatidic acid (PA), activates PAK1-mediated RhoGDI phosphorylation on Ser-101/174, causing selective Rac1 release and activation. Phosphorylation of RhoGDI on Ser-34 by PKCa has recently been demonstrated to selectively release RhoA, promoting RhoA activation. Here, I show DGKz is required for optimal RhoA activation and RhoGDI Ser-34 phosphorylation. Both were substantially reduced in DGKz-null fibroblasts and occurred independently of DGKz activity, but required a function DGKz PDZ-binding motif. In contrast, Rac1 activation required DGKz-derived PA, but not PDZ-interactions, indicating DGKz regulates these Rho GTPases by two distinct regulatory complexes. Interestingly, RhoA bound directly to the DGKz C1A domain, the same region known to bind Rac1. By direct interactions with RhoA and PKCa, DGKz was required for the efficient co-precipitation of these proteins, suggesting it is important to assemble a signalling complex that functions as a RhoA-specific RhoGDI dissociation complex. Consequently, cells lacking DGKz exhibited decreased RhoA signalling downstream and disrupted stress fibers. Moreover, DGKz loss resulted in decreased stress fiber formation following the expression of a constitutively active RhoA mutant, suggesting it is also important for RhoA function following activation. This is consistent with the ability of DGKz to bind both active and inactive RhoA conformations. Collectively, these findings suggest DGKz is central to two distinct Rho GTPase activation complexes, each having different requirements for DGKz activity and PDZ interactions, and might regulate the balance of Rac1 and RhoA activity during dynamic changes to the actin cytoskeleton.