Das Vasodilatator stimulierte Phosphoprotein (VASP) ist ein Zytoskelett-assoziiertes Protein der Ena (Enabled)/VASP-Proteinfamilie. Seine Funktionen bezüglich Aktin- Polymerisation, Thrombozyten-Aggregation, Wachstumskegel-Führungsprozessen und Motilität sowohl von Zellen als auch von Listerien sind bisher nur unvollständig charakterisiert. In dieser Arbeit konnte ich zeigen, wie die VASP-F-Aktin Interaktion in vitro durch die Phosphorylierung zweier Aminosäuren von VASP reguliert wird. Transfektions-Experimente mit VASP und RhoA deuten eine mögliche Beteiligung von VASP im Signalweg von RhoA an. Zudem führen Überexpression und Deletion von VASP in Zellen zu demselben Stressfaser-Phänotyp, der unabhängig vom stimulierenden Einfluss von Serum ist. In VASPdefizienten Fibroblasten ist außerdem die Membranrigidität und die Phosphorylierung der leichten Kette des Myosins erhöht, was auf ein stabileres und stärker kontrahiertes Aktin- Zytoskelett in diesen Zellen schließen lässt. Die Regulation und Organisation des Aktin- Zytoskeletts beeinflusst auch die zelluläre Adhäsion, die in VASP-defizienten Zellen verändert ist. VASP-defiziente Zellen adhärieren signifikant stärker an Fibronektinbeschichtete Perlen als Wildtyp-Zellen. Der Widerstand dieser Perlen gegenüber mechanischen Kräften ist in VASP (-/-) Zellen signifikant erhöht. Dieser Unterschied beruht in erster Linie auf dem verstärkten Aktin-Zytoskelett in diesen Zellen und ist unabhängig von Mikrotubuli. Messungen mit rekonstituierten Zelllinien zeigen zudem eine VASPAbhängigkeit dieses Effekts. Der GTPase Rap1 kommt eine wichtige Bedeutung bei der Integrin-abhängigen Adhäsion von Zellen zu. Aktivierung von Epac, einem Guanin-Nukleotid- Austauschfaktor von Rap1, führt in Wildtyp-Zellen zur Verstärkung des Widerstandes gegenüber mechanischen Kräften, die auf Fibronektin-beschichtete Perlen wirken, ohne dabei die Membranrigidität zu verändern. Diese Kraft-Verstärkung wird weder vom Aktin- noch vom Mikrotubuli-Zytoskelett beeinflusst. Es exisitiert daher ein Mechanismus, bei dem die Länge von elastischen Membranfortsätzen unabhängig von der Membranfestigkeit und dem Aktin-Zytoskelett reguliert wird. Diese Experimente zeigen, dass VASP eine wichtige Rolle bei der Stabilität und Kontraktilität des Aktin-Zytoskeletts, der Membranrigidität sowie bei der zellulären Adhäsion spielt. Die vorliegenden Ergebnisse weisen darauf hin, dass hierbei weniger die direkte Interaktion von VASP mit dem Aktin-Zytoskelett von Bedeutung ist, sondern viel mehr seine mögliche Funktion als Gerüstprotein (Scaffold), das eine geregelte Signaltransduktion organisiert. / The actin-cytoskeleton associated protein VASP (vasodilator stimulated phosphoprotein) is a founding member of the Ena (enabled)/VASP protein family. Its function in actinpolymerisation, platelet aggregation, axon-guidance, cell motility and Listeria monocytogenes is yet not completely understood. In this work I could show that the phosphorylation of two amino acid residues of VASP diminishes its interaction with F-actin. Transfection experiments with VASP and RhoA suggest a possible role for VASP in the RhoA signalling pathway. Both, overexpression and deletion of VASP results in a similar stressfiber phenotype, which is independent of the stimulatory effect of serum. Furthermore membrane rigidity and myosin light chain phosphorylation in VASP deficient cells is increased, indicating a more contracted and rigid cytoskeleton in these cells. Cell adhesion is influenced by the regulation and organisation of the actin-cytoskeleton. VASP deficient cells not only adhere significantly stronger to fibronectin-coated beads but also resistance to displacement by mechanical forces is enhanced. This effect results from greater stability of the actin-cytoskeleton while there is no evidence for microtubuli involvement. Experiments with reconstituted VASP (-/-) cell lines proof this effect being VASP-dependent. The GTPase Rap1 is an important regulator of integrin-dependent cell adhesion. Activation of Epac, a guanine nucleotid exchange factor for Rap1, in wildtyp cells leads to enhanced resisting forces to mechanical displacement of fibronectin-coated beads while membrane rigidity remains unchanged. This phenomenon is neither dependent on F-actin nor microtubuli indicating a modulation of membrane tethers independently of membrane rigidity. These results show that VASP is an important regulator of actin-cytoskeletal stability and contractility, as well as membrane rigidity and cell adhesion. Furthermore these data suggest a possible role for VASP as a scaffold protein organising intracellular signal transduction.

Thesis advisor: David R. Burgess / Establishment and maintenance of cell polarity has become an increasingly interesting biological question in a diversity of cell types and has been found to play a role in a variety of biological functions. Previously, it was thought that the echinoderm embryo remained relatively unpolarized until the first asymmetric division at the 16 cell stage of development. However, there is mounting evidence to suggest that polarity is established much earlier. I analyzed roles of the cell polarity regulators, the PAR complex proteins, and how their disruption in early development affects later developmental milestones such as blastula formation. I found that PAR6 along with aPKC and CDC42 localize to the apical cortex (free surface) as early as the 2 cell stage of development and this localization is retained through the gastrula stage. Interestingly, PAR1 also colocalizes with these apical markers through the gastrula stage, despite the formation of a polarized epithelium and a series of asymmetric divisions. Additionally, PAR1 was found to be in complex with aPKC, but not PAR6, during these developmental stages. PAR6, aPKC, and CDC42 are anchored in the cortex by assembled myosin; however, a clear role for myosin assembly in PAR1 localization could not be determined. Furthermore, myosin assembly was found to be necessary to maintain proper PAR6 localization through subsequent cleavage divisions. Interference with myosin assembly prevented the embryos from reaching the blastula stage, while transient disruptions of either actin or microtubules did not have this effect. Similarly, inhibition of aPKC activity during early cleavage stages impeded blastula formation; however, aPKC is not involved in the regulation of the first asymmetric division at the 16 cell stage in sea urchin embryos. These observations suggest that disruptions of the polarity complex in the early embryo can have a significant impact on the ability of the embryo to reach later critical stages in development. / Thesis (PhD) — Boston College, 2014. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Biology.

Cell migration is essential for embryonic development, it occurs in adult organisms during processes like wound healing and its misregulation contributes to pathological conditions such as metastasis. Despite this, most studies of cell migration have been undertaken in vitro. Ena/VASP proteins, believed to be actin anti-capping proteins, have been studied extensively in fibroblasts in vitro, and using Drosophila macrophages (hemocytes) within the developing embryo, the role of the Drosophila homologue of Mena, Ena, is investigated in vivo. Consistent with data from fibroblasts in vitro, Ena localised to regions of actin dynamics within migratory hemocytes, where this protein stimulated lamellipodial dynamics and positively regulated filopodial number and length. However, whilst overexpression of Ena/VASP proteins in fibroblasts reduced migration speeds, Ena overexpression in hemocytes dramatically increased migration speeds in three different assays. This positive regulation of migration speed closely resembled the increased motility of breast cancer cells that overexpress Mena and evidence presented here, suggests that this key difference may be explained by spatial constraints that are imposed upon cells within three dimensional environments. Indeed, such constraints prevented ruffling, a more detrimental form of retraction, in hemocytes in vivo. Furthermore, fibroblasts overexpressing Mena in vitro form membrane ruffles more frequently. Therefore Ena/VASP proteins drive migration by enhancing lamellipodial protrusion, but in certain environments these protrusions are lost as ruffles slowing migration. The method by which Ena regulates lamellipodial protrusion and migration speeds was then investigated: Ena increased Fascin-mediated actin bundling and the number of Fascin rich-actin bundles that coalesced. Analysis of individual actin bundles revealed that coalescence increased protrusion rate and that both protrusion rate and coalescence, increased cell migration speeds. This suggests that Ena facilitates an increase in cell migration by promoting the coalescence of Fascin bundles, and positions Ena as a key regulator of migration speeds in vivo.

Streptococcus pneumoniae is one of the major causes of bacterial meningitis, which mainly affects young infants in the developing countries of Africa, Asia (esp. India) and South America, and which has case fatality rates up to 50% in those regions. Bacterial meningitis comprises an infection of the meninges and the sub-meningeal cortex tissue of the brain, whereat the presence of pneumolysin (PLY), a major virulence factor of the pneumococcus, is prerequisite for the development of a severe outcome of the infection and associated tissue damage (e. g. apoptosis, brain edema, and ischemia). Pneumolysin belongs to the family of pore forming, cholesterol-dependent cytolysins (CDCs), bacterial protein toxins, which basically use membrane-cholesterol as receptor and oligomerize to big aggregates, which induce cell lysis and cell death by disturbance of membrane integrity. Multiple recent studies, including this work, have revealed a new picture of pneumolysin, whose cell-related properties go far beyond membrane binding, pore formation and the induction of cell death and inflammatory responses. For a long time, it has been known that bacteria harm the tissues of their hosts in order to promote their own survival and proliferation. Many bacterial toxins aim to rather hijack cells than to kill them, by interacting with cellular components, such as the cytoskeleton or other endogenous proteins. This study was able to uncover a novel capacity of pneumolysin to interact with components of the actin machinery and to promote rapid, actin-dependent cell shape changes in primary astrocytes. The toxin was applied in disease-relevant concentrations, which were verified to be sub-lytic. These amounts of toxin induced a rapid actin cortex collapse in horizontal direction towards the cell core, whereat membrane integrity was preserved, indicating an actin severing function of pneumolysin, and being consistent with cell shrinkage, displacement, and blebbing observed in live cell imaging experiments. In contrast to neuroblastoma cells, in which pneumolysin led to cytoskeleton remodeling and simultaneously to activation of Rac1 and RhoA, in primary astrocytes the cell shape changes were seen to be primarily independent of small GTPases. The level of activated Rac1 and RhoA did not increase at the early time points after toxin application, when the initial shape changes have been observed, but at later time points when the actin-dependent displacement of cells was slower and less severe, probably presenting the cell’s attempt to re-establish proper cytoskeleton function. A GUV (giant unilamellar vesicle) approach provided insight into the effects of pneumolysin in a biomimetic system, an environment, which is strictly biochemical, but still comprises cellular components, limited to the factors of interest (actin, Arp2/3, ATP, and Mg2+ on one side, and PLY on the other side). This approach was able to show that the wildtype-toxin, but not the Δ6 mutant (mutated in the unfolding domain, and thus non-porous), had the capacity to exhibit its functions through a membrane bilayer, meaning it was able to aggregate actin, which was located on the other side of the membrane, either via direct interaction with actin or in an Arp2/3 activating manner. Taking a closer look at these two factors with the help of several different imaging and biochemical approaches, this work unveiled the capacity of pneumolysin to bind and interact both with actin and Arp2 of the Arp2/3 complex. Pneumolysin was capable to slightly stabilize actin in an actin-pyrene polymerization assay. The same experimental setup was applied to show that the toxin had the capacity to lead to actin polymerization through activation of the Arp2/3 complex. This effect was additionally confirmed with the help of fluorescent microscopy of rhodamine (TRITC)-tagged actin. Strongest Arp2/3 activation, and actin nucleation/polymerization is achieved by the VCA domain of the WASP family proteins. However, addition of PLY to the Arp2/3–VCA system led to an enhanced actin nucleation, suggesting a synergistic activation function of pneumolysin. Hence, two different effects of pneumolysin on the actin cytoskeleton were observed. On the one hand an actin severing property, and on the other hand an actin stabilization property, both of which do not necessarily exclude each other. Actin remodeling is a common feature of bacterial virulence strategies. This is the first time, however, that these properties were assigned to a toxin of the CDC family. Cytoskeletal dysfunction in astrocytes leads to dysfunction and unregulated movement of these cells, which, in context of bacterial meningitis, can favor bacterial penetration and spreading in the brain tissue, and thus comprises an additional role of pneumolysin as a virulence factor of Streptococcus pneumonia in the context of brain infection. / S. pneumoniae gehört zur Gruppe der Pathogene, die bakterielle Meningitis verursachen, eine Infektion, die hauptsächlich bei Neugeborenen und Kleinkindern in den Entwicklungsländern von Afrika, Asien und Südamerika auftritt, und in diesen Regionen Sterblichkeitsraten von bis zu 50% aufweist. Meningitis ist eine Infektion der Hirnhäute und dem sich direkt darunter befindlichen Cortex-Gewebe. Pneumolysin (PLY), ein Haupt-Pathogenitätsfaktor des sog. Pneumococcus, ist hauptsächlich verantwortlich für einen schweren Verlauf der Infektion und für Gewebeschädigungen, wie Apoptose, Hirnödemen und Ischämie. Pneumolysin gehört zur Familie der Cholesterol-abhängigen Cytolysine (CDCs), bakteriellen Protein-Toxinen, die an Membran-Cholesterol binden, sich zu großen Aggregaten zusammenschließen und durch die Beeinträchtigung der Membranintegrität (Porenbildung) Zell-Lyse und Zelltod verursachen. Zahlreiche neuere Studien, darunter auch diese Arbeit, haben ein neues Bild von Pneumolysin aufgezeigt, dessen Eigenschaften weit über die Membranbindung, die Poren-Bildung und die Induktion von Zelltod und inflammatorischen Prozessen hinausgehen. Es ist weithin bekannt, dass Bakterien das Gewebe ihres Wirts schädigen, um ihre eigene Vermehrung und ihre Ausbreitung zu begünstigen. In diesem Zusammenhang fungieren bakterielle Toxine als Pathogenitätsfaktoren, die mit zellulären Komponenten, wie dem Zytoskelett und anderen Zytosol-Proteinen interagieren, was allerdings bevorzugt zu Zellveränderungen, und seltener zum Zelltod führt. Die vorliegende Arbeit konnte zeigen, dass Pneumolysin schnelle, und zum Teil gravierende, Aktin-abhängige Zellstruktur-Veränderungen in primären Astrozyten hervorruft. Hierbei wurde das Toxin in Konzentrationen appliziert, die im Liquor von Meningitis-Patienten detektiert werden können, und die zusätzlich als sub-lytisch für Astrozyten in Zellkultur verifiziert wurden. Diese Toxin-Mengen führten zu einem schnellen, horizontalen Aktinkortex-Kollaps, wobei die Membranintegrität erhalten blieb. Dies deutete auf eine „Severing“-Funktion (das Abtrennen oder Zerschneiden von Aktinfilamenten) von Pneumolysin hin, was mit den Beobachtungen übereinstimmt, die in Experimenten mit lebendigen Zellen gemacht wurden (Zellveränderungen, Zellbewegungen und „Blebbings“). Im Gegensatz zu Neuroblastoma Zellen, in denen Pneumolysin Zytoskelett-Veränderungen, und gleichzeitig die Aktivierung von Rac1 und RhoA verursachte, waren die Zell-Veränderungen bei Astrozyten primär unabhängig von der Aktivierung kleiner GTPasen. Obwohl gezeigt werden konnte, dass die Veränderungen vom Aktin-Zytosklett abhängig waren, war das Level an Rac1 und RhoA in den frühen Phasen nach der Toxin-Gabe nicht erhöht. Eine Aktivierung der GTPasen konnte dahingegen zu späteren Zeitpunkten detektiert werden, in denen die Zellbewegung abgeschwächt und verlangsamt war. Die späte Aktivierung kann als Reaktion der Zelle auf die vom Toxin ausgelösten Veränderungen gesehen werden, die zu einer Wiederherstellung der normalen Zytoskelett-Funktion führen soll. GUV-Experimente ermöglichten eine genauere Betrachtung der Pneumolysin-Effekte in einem biomimetischen, jedoch strikt biochemischen Ansatz, der alle zellulären Komponenten enthält, die untersucht werden sollen (Pneumolysin, Aktin, Arp2/3, ATP, und Mg2+). Im GUV-System befand sich das Toxin im Inneren der Vesikel, und Aktin in der extra-vesikulären Suspension, einem Verhältnis genau umgekehrt zum zellullären System. Zusätzlich wurden Arp2/3 und ATP/Mg2+, für die Aktin-Polymerisierung essentielle Faktoren, in der Aktin-Suspension zur Verfügung gestellt. Die GUV-Experimente konnten zeigen, dass Wildtyp-Pneumolysin, allerdings nicht seine Mutante Δ6-PLY (Mutation in der sog. unfolding domain, und deshalb nicht Poren-bildend), seine Effekte auf das Aktin-Zytoskelett durch die Membran-Barriere hindurch, in einer Membran-gebundenen Form ausüben kann. Aktin wurde an den Stellen höchster Toxinbindung aggregiert, was entweder über eine direkte Interaktion von PLY mit Aktin, oder über eine Aktivierung des Aktin-Effektors Arp2/3 durch Pneumolysin erklärt werden kann. Weitere biochemische Ansätze (wie enzyme-linked sorbent assays, ELSAs) und Mikroskopie-Techniken (Immunocyto-Chemie) konnten beweisen, dass Pneumolysin sowohl mit Aktin, als auch mit Arp2 (einer Komponente des heptameren Arp2/3 Proteinkomplexes) direkt interagieren kann. Aktin-Pyren Experimente und Fluoreszenzmikroskopie (von TRITC-markiertem Aktin) wiesen auf eine Kapazität von Pneumolysin hin, Aktin direkt zu stabilisieren, und über die Aktivierung von Arp2/3 eine Aktin-Polymerisierung hervorrufen zu können.

It is becoming clear that mechanical stimuli are critical in regulating cell biology; however, the short-term structural response of a cell to mechanical forces remains relatively poorly understood. We mechanically stimulated cells expressing actin-EGFP with controlled forces (0-20nN) in order to investigate the cell’s structural response. Two clear force dependent responses were observed: a short-term local deformation of actin stress fibres and a long-term force-induced remodelling of stress fibres at cell edges, far from the point of contact. We were also able to quantify strain dynamics occurring along stress fibres. The cell exhibits complex heterogeneous negative and positive strain fluctuations along stress fibres, indicating localized dynamic contraction and expansion. A ~50% increase in myosin contractile activity is apparent following application of 20nN force. Directly visualizing force-propagation and stress fibre strain dynamics has revealed new information about the pathways involved in mechanotransduction which ultimately govern the downstream response of a cell.

It is becoming clear that mechanical stimuli are critical in regulating cell biology; however, the short-term structural response of a cell to mechanical forces remains relatively poorly understood. We mechanically stimulated cells expressing actin-EGFP with controlled forces (0-20nN) in order to investigate the cell’s structural response. Two clear force dependent responses were observed: a short-term local deformation of actin stress fibres and a long-term force-induced remodelling of stress fibres at cell edges, far from the point of contact. We were also able to quantify strain dynamics occurring along stress fibres. The cell exhibits complex heterogeneous negative and positive strain fluctuations along stress fibres, indicating localized dynamic contraction and expansion. A ~50% increase in myosin contractile activity is apparent following application of 20nN force. Directly visualizing force-propagation and stress fibre strain dynamics has revealed new information about the pathways involved in mechanotransduction which ultimately govern the downstream response of a cell.

Cancer remains one of the most common causes for death in the world today. Researchers are continuously trying to improve old, and develop new, methods in order to strife this global problem. Much research is being made trying to find new specific biomarkers that can be used to detect and diagnose cancer in an early stage. One candidate protein for possible future use as a biomarker is tripeptidyl-peptidase II (TPPII) which has previously been shown to be up-regulated in Burkitt´s lymphoma. This paper focuses on the expression of TPPII on an mRNA-level to see if there is any difference between expression in human leucocytes from patients with a leukemia diagnosis and a healthy volunteer, in order to evaluate if the expression of TPPII have any future use as a biomarker. Patient samples were analyzed using real time qPCR, to study the expression of mRNA, and Western blot, in order to correlate the mRNA findings with protein expression. Three different cell lines with different characteristics regarding expression and function of TPPII were also used to validate the methods used and for comparison with the patient samples analyzed. A difference in expression of mRNA were seen between the different patient samples, both individually and between larger groups of samples with the same diagnosis, indicating a large individual variation, thus making future use in a clinical setting difficult. However, seeing as only a few samples were analyzed in this study, more research must be done in order to draw any final conclusions.

TRiC (TCP-1 ring complex), a type II chaperonin, facilitates protein folding, and we previously showed that TRiC crosslinks to ribosome-bound actin and luciferase nascent chains. Here, it was found that actin and luciferase nascent chains were adjacent to more than one TRiC subunit at different stages of translation. Six and seven out of the eight TRiC subunits were photocrosslinked to the luciferase and actin nascent chains, respectively. Actin nascent chains with widely-spaced, site-specific probe locations were adjacent to the same three TRiC subunits (a, b and e) at different stages of translation. The exposure of other TRiC subunits to nascent chains varied with the length and identity of the nascent chain. In addition, the presence or absence of ATP influences the photocrosslinking yields. This suggests that ATP alters the conformation of the subunits and/or their affinity for the nascent chain. Photocrosslinking also revealed that TRiC is in close proximity to the exit site of the ribosomal tunnel, presumably to create a protected folding environment for the nascent chain. Immunoprecipitations under native conditions revealed that prefoldin photocrosslinks to the actin nascent chain and that these prefoldin-containing photoadducts are coimmunoprecipitated with antibodies specific for the TRiC a subunit. This result suggests that prefoldin and TRiC bind simultaneously to the same actin nascent chain. Photocrosslinking studies with probes at position 68 in the actin nascent chain revealed that prefoldin binds to the nascent chain subsequently to TRiC binding. An unknown protein with an apparent molecular mass of 105 kDa was shown to photocrosslink to the luciferase nascent chain in a length-dependent manner at specific probe locations close to the N-terminus of the nascent chain. Thus, the nascent chain sees a variety of proteins in its immediate environment as it emerges from the ribosomal tunnel and undergoes its chaperonin-assisted folding.

A fundamental yet unresolved issue in cell biology is how force regulates actin dynamics and how this biophysical regulation is modulated by biochemical signaling molecules. Here we show, by atomic force microscopy (AFM) force-clamp experiments, that tensile force regulates the kinetics of G-actin/G-actin and G-actin/F-actin interactions by decelerating dissociation at low forces (catch bonds) and accelerating dissociation at high forces (slip bonds). The catch bonds can be structurally explained by force-induced formation of new interactions between actin subunits (Steered molecular dynamics (SMD) simulations performed by Dr. Jizhong Lou, Institute of Biophysics, Chinese Academy of Sciences, Beijing, China). K113S mutation on yeast actin suppressed the actin catch-slip bonds, supporting the structural mechanism proposed by SMD simulations. Moreover, formin controlled by a RhoA-mediated auto-inhibitory module can serve as a "molecular switch", converting the catch-slip bonds to slip-only. These results imply anisotropic stability of the actin network in cells subjected to directional forces, possibly explaining force-induced cell and actin fiber alignment controlled by RhoA and formin. Our study suggests a molecular level crosstalk mechanism bridging the actin-mediated mechanotransduction and biochemical signal transduction pathways.

Thesis (M.S.)--West Virginia University, 2008. / Title from document title page. Document formatted into pages; contains ix, 115 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 103-105).