The carcinoembryonic antigen-related cellular adhesion molecule 1 (CEACAM1) is a human receptor that facilitates adhesion with neighbouring cells, as well as with certain pathogens. CEACAM1 at the cell surface exists as a mixture of monomers and dimers in a heterogeneous distribution that is thought to regulate the balance of its functions, including those associated with pathogen binding. We used live cell fluorescence and homogeneous Förster resonance energy transfer (homo-FRET) microscopy on a combined total internal reflection fluorescence polarization (TIRFPM) confocal microscopy platform to investigate the distribution, dynamics, and monomer-dimer equilibrium of CEACAM1-4L-EYFP on live cells that were parachuted onto surfaces coated with CEACAM1-binding Neisseria gonorrhoea. Both CEACAM1-4L-EYFP and a monomeric mutant form of the receptor are rapidly recruited to bacteria and lead to downstream effector recruitment. Homo-FRET data indicate that wild-type CEACAM1-4L-EYFP was predominantly monomeric at bacterial contact sites. Preferential monomeric binding during bacterial adhesion controls the infection process.

The carcinoembryonic antigen-related cellular adhesion molecule 1 (CEACAM1) is a human receptor that facilitates adhesion with neighbouring cells, as well as with certain pathogens. CEACAM1 at the cell surface exists as a mixture of monomers and dimers in a heterogeneous distribution that is thought to regulate the balance of its functions, including those associated with pathogen binding. We used live cell fluorescence and homogeneous Förster resonance energy transfer (homo-FRET) microscopy on a combined total internal reflection fluorescence polarization (TIRFPM) confocal microscopy platform to investigate the distribution, dynamics, and monomer-dimer equilibrium of CEACAM1-4L-EYFP on live cells that were parachuted onto surfaces coated with CEACAM1-binding Neisseria gonorrhoea. Both CEACAM1-4L-EYFP and a monomeric mutant form of the receptor are rapidly recruited to bacteria and lead to downstream effector recruitment. Homo-FRET data indicate that wild-type CEACAM1-4L-EYFP was predominantly monomeric at bacterial contact sites. Preferential monomeric binding during bacterial adhesion controls the infection process.

No description available.

The molecular cloning and subsequent engineering of the green fluorescent protein (GFP) of the jellyfish Aequoria victoria allowed a novel approach to the investigation of cell signalling. GFP and its mutants can now not only be used to target specific organelles in living cells but also function as a basis for a variety of sensors for biologically important ions and molecular interactions. GFP-based Ca2+- sensors have been successfully used for studies in mammalian and plant cells. In filamentous fungi, however, they have not yet been reported to work. Since only little is known about calcium signalling in filamentous fungi, this project aimed to improve existing GFP-based Ca2+- sensors by exchanging the original fluorophores for improved versions and expressing those in the filamentous fungus Aspergillus niger. During this project, the donor and acceptor fluorophores of 3 existing Ca2+-FRETprobes based on cameleons and troponin C-sensors, have been changed, 2 novel positive FRET controls have been designed and these , as well as donor and acceptor fluorophores alone, have been expressed in the filamentous fungus Aspergillus niger. The probes were assessed using different imaging techniques, such as conventional confocal laser scanning microscopy (CLSM), fluorescence lifetime imaging microscopy (FLIM) and spectral imaging using a Leica TSC SP5 confocal and IRIS, a novel spectral imaging device designed at Heriot Watt University. Problems were encountered that prevented FRET analysis using CLSM and IRIS. These were due mainly to the difference in expression level of the constructs and the distribution of the emission bandpasses of the IRIS system. Analysis of the spectral data obtained on the Leica confocal system and analysis of the FLIM results, however, revealed significant differences between the donor only and the positive FRET controls. Spectra of the positive FRET controls and the Ca2+-sensitive probes showed emission peaks of both the donor and the acceptor fluorophores upon excitation of the donor fluorophore alone while analysis of the FLIM results revealed an additional decay component in the positive FRET controls. Both results are very strong indicators that we can detect FRET in living hyphae of Aspergillus niger transformed with the probes designed during this project.

This thesis describes the dynamics of mitochondrial RNA-binding protein 1 and 2 (MRP1/2) complex in different cell lines of Trypanosoma brucei under an optimized immobilized condition. This study reveals the influence of RNA on the complex's dynamics. Furthermore, the function of RNA-binding complex 1 (MRB1) core has been studied via reverse genetic, biochemical and molecular techniques, with its role in RNA editing being proposed.

Live cell imaging is a unique tool for cellular research with a wide variety of applications. By streaming digital microscopic images an investigator can observe the dynamic morphology of a cell, track cell movement on a surface, and measure quantities or localization patterns of fluorescently labeled proteins or molecules. Digital image sequences contain a vast amount of information in the form of visually detectable morphological changes in the cell. We designed computer programs that allow the manual identification of visible events in live cell digital image sequences [Davis et al. 2007]. Once identified, the data are analyzed using algorithms to calculate the yield of individual events per cell over the time course of image acquisition. The sequence of event data is also constructed into directed acyclic graphs and through the use of a subgraph isomorphism algorithm we are able to detect specified patterns of events originating from a single cell. Two projects in the field of cancer research are here discussed that describe and validate the application of the event analysis programs. In the first project, mitotic catastrophe (MC) research [Ianzini and Mackey, 1997; Ianzini and Mackey, 1998; reviewed by Ianzini and Mackey, 2007] is enhanced with the addition of live cell imaging to traditional laboratory experiments. The event analysis program is used to describe the yield of normal or abnormal divisions, fusions, and cell death, and to detect patterns of reductive division and depolyploidization in cells undergoing radiation-induced MC. Additionally, the biochemical and molecular data used in conjunction with live cell imaging data are presented to illustrate the usefulness of combining biology and engineering techniques to elucidate pathways involved in cell survival under different detrimental cell conditions. The results show that the timing of depolyploidization in MC cells correlates with increased multipolar divisions, up-regulation of meiosis-specific genes, and the production of mononucleated cell progeny. It was confirmed that mononucleated cells are produced from multipolar divisions and these cells are capable of resuming normal divisions [Ianzini et al., 2009]. The implications for the induction of meiosis as a mechanism of survival after radiation treatment are discussed. In the second project, the effects of long-term fluorescence excitation light exposure are examined through measurements of cell division and cell death. In the field of live cell imaging, probably the most modern and most widely utilized technique is fluorescence detection for intracellular organelles, proteins, and molecules. While the technologies required to label and detect fluorescent molecules in a cell are well developed, they are not idealized for long term measurements as both the probes and excitation light are toxic to the cells [Wang and Nixon, 1978; Bradley and Sharkey, 1977]. From the event analysis data it was determined that fluorescence excitation light is toxic to multiple cell lines observed as the reduction of normal cell division, induction of cell death, and apparent morphological aberrations.

La transcription est une étape fondamentale dans l'expression des gènes. Cependant, elle reste incomplètement caractérisée dans les cellules vivantes. Pour mieux comprendre la dynamique de la transcription, notre laboratoire a amélioré le système de marquage d'ARN en utilisant la séquence codante pour MS2, facilement fusionnée avec le promoteur d'intérêt et inséré copie unique dans deux lignées cellulaires HeLa cellules. Cette construction permet une vue quantitative de la transcription, a l’échelle de la molécule unique, en temps réel. Nous avons trouvé que le VIH-1 est transcrit par des groupes de polymérases nominés convois. La transcription oscille de manière aléatoire avec des périodes actives (ON) et inactives (OFF) et est contrôlée indépendamment.Sur la base de cette découverte, nous avons étudié: (i) comment l'architecture de différents promoteurs de mammifères contrôle la cinétique transcriptionnelle; et (ii) le rôle du transactivateur transcriptionnel (Tat), le régulateur principal de la transcription du VIH-1, dans les cellules vivantes. Pour traiter ces questions, une nouvelle méthode de modélisation a été établi, combinant l'information des fluctuations transcriptionnelles avec différentes résolutions temporelles. Cela a donné une vue complète et précise du processus stochastique, décrit par le modèle de Markov. Cinq des six promoteurs de mammifères pourraient être définis par trois états, probablement contrôlés par des mécanismes différents. Le passage entre ces états est défini par les constantes de vitesse et l'écart entre eux pourrait potentiellement expliquer la différence dans la quantité d'ARN produit. De manière intéressante, nous avons constaté que les taux de passage entre les états inactifs et profondément silencieux sont la marque distinctive de différents promoteurs, suggérant que les événements cruciaux définissant les profils transcriptionnels sont en fait des événements pré-transcriptionnels.Pour étudier le rôle de Tat, des lignées cellulaires contenant un rapporteur du VIH-1 et une quantité différente de Tat ont été produites. Avec cette approche décrite ci-dessus, nous avons montré que Tat, précédemment caractérisé en tant qu'acteur dominant dans la libération de la polymérase en pause, agit longtemps avant que la transcription soit initiée. Ces résultats frappants apportent de nouvelles perspectives concernant la dynamique transcriptionnelle du VIH-1 contrôlée par Tat. / Transcription is a fundamental step in gene expression. However, it is incompletely characterized in single living cells. To address this question, our laboratory developed the improved RNA tagging system using MS2-binding protein that could easily be fused with the promoter of interest inserted in a single copy in HeLa cell lines. This construct allows quantitative, single molecule view of the transcription in a real time. We have found that HIV-1 is transcribed by groups of closely spaced polymerases referred as convoys. The transcription oscillates randomly between active (ON) and inactive (OFF) periods that are controlled independently.On the basis of this discovery, we further investigated: (i) how architecture of different mammalian promoters controls the transcriptional kinetics; and (ii) the role of transcriptional transactivator (Tat), the master regulator of in HIV-1 transcription in living cells. To address this, new pipeline for the quantification was established, combining the information of transcriptional fluctuations with different temporal resolutions. This gave the full and precise view of the stochastic switching, described by the Markov model. Five of six mammalian promoters could be defined by three states, probably controlled by different mechanisms. Switching between them is defined by the rate constants and the discrepancy among them could potentially explain the difference in the amount mRNA produced. Interestingly, we found that switching rates between inactive, deeply silent states are the hallmark of different promoters, suggesting that the crucial events defining the transcriptional profiles are in fact pre-transcriptional events.To address the role of Tat, cell lines containing HIV-1 reporter and different amounts of Tat were produced. With the above described approach, we found that Tat, previously characterized as dominant player in the release of the paused polymerase, actually acts long before the transcription is initiated. These striking results bring new insights of HIV-1 transcriptional dynamics controlled by Tat.

No description available.

Plant viruses have developed a number of strategies that enable them to become obligate intracellular parasites of many agricultural crops. Potato virus X (PVX) belongs to a group of positive-sense, single-stranded plant RNA viruses that replicate on host membranes and form elaborate structures known as viral replication complexes (VRCs) that contain viral RNA (vRNA), proteins and host cellular components. VRCs are the principal sites of viral genome replication, virion assembly and packaging of vRNA for export into neighbouring cells. For many animal viruses, host membrane association is crucial for RNA export. For plant viruses, it is not yet known how vRNA is transported to and through plant plasmodesmata. PVX encodes genetic information required for its movement between cells; three viral triple gene block (TGB) movement proteins and a viral coat protein are essential for viral trafficking. This research project studies the relationship between PVX and its host plants, Nicotiana benthamina and Nicotiana tabacum. A particular focus of this project is exploration of the structural and functional significance of the PVX VRC and how the virus recruits cell host components for its replication and movement between cells. The role of specific viral proteins in establishing the VRC, and the ways in which these interact with host organelles, was investigated. A combination of different approaches was used, including RNA-binding dyes and a Pumilio-based bimolecular fluorescence complementation assay for detection of the vRNA, fluorescent reporters for virusencoded proteins, fluorescent reporters for host organelles involved in viral replication, and also transgenic tobacco plants expressing reporters for specific plant components (endoplasmic reticulum, Golgi, actin, microtubules and plasmodesmata). In addition, mutagenesis was used to study the functions of individual viral proteins in replication and movement. All of these approaches were combined to achieve live-cell imaging of the PVX infection process. The PVX VRC was shown to be a highly compartmentalised structure; (+)-stranded vRNA was concentrated around the viral TGB1 protein, which was localised in discrete circular compartments within the VRC while coat protein was localised to the external edges of the VRC. The vRNA was closely associated with host components (endoplasmic reticulum and actin) shown to be involved in the formation of the VRC. The TGB2/TGB3 viral proteins were shown to colocalise with the host endomembranes (ER) and to exit these compartments in the form of motile granules. vRNA, TGB1, TGB2 and CP localised to plasmodesmata of the infected cells. TGB1 was shown to move cell-to-cell and recruit ER, Golgi and actin in the absence of viral infection. In the presence of virus, TGB1 targeted the VRCs in several neighbouring cells. A model of PVX replication and movement is proposed in which TGB1 functions as a key component for recruitment of host components into the VRC to enable viral replication and spread.

The primary research aims of this thesis were to analyse the mechanism of Ca2+/calmodulin (CaM) signalling during conidial germination and conidial anastomosis tube (CAT)-mediated fusion in Neurospora crassa. Ca2+ is an ubiquitous signalling molecule that regulates many important processes in filamentous fungi including spore germination, hyphal growth, mechanosensing, stress responses, circadian rhythms, and the virulence of pathogens. Transient increases in cytosolic free calcium ([Ca2+]c) act as intracellular signals. As the primary intracellular Ca2+ receptor, calmodulin (CaM) converts these Ca2+ signals into responses by regulating the activities of numerous target proteins. Ca2+-free medium, antagonists of L-type Ca2+ channels, CaM and calcineurin were found to inhibit CAT fusion. In addition, my results showed that CAT chemotropism is dependent on extracellular Ca2+. 65 genes were identified as likely components of the Ca2+ signalling machinery of N. crassa based on a comparative genomic analysis of S. cerevisiae, A. fumigatus and C. albicans. Deletion mutants of 29 of these genes were characterized in relation to their possible roles during colony initiation and development. Four of these mutants (Δcna-1, Δcnb-1, Δcamk-1, Δplc-2, and Δrgs-1), which were homokaryons, exhibited strong morphological phenotypes associated with CAT fusion. To identify the protein machinery involved in Ca2+/CaM signalling during colony initiation, proteins that directly or indirectly interacted with CaM were isolated from germlings by immunoprecipitation and analyzed by mass spectroscopy. A total of 286 putative Ca2+/CaM-interacting proteins were identified in this way and 30 of these proteins contained CaM-binding motifs. This proteomics analysis provided evidence for Ca2+/CaM signalling playing a role in regulating the activity of a wide range of proteins including MAP kinases in the cell integrity pathway, Ras/Rho signalling pathway, and microtubule and actin cytoskeletal proteins. GFP labelled CaM localized as dynamic spots associated with the plasma membrane and cytoplasm in both germ tubes and CATs. Significant CaM accumulation was observed in the tips of CATs growing towards each other, around fusion pores at sites of CAT fusion, and at developing septa in germ tubes. CaM localization was influenced by the actin and microtubule cytoskeleton during the colony initiation. Inhibition of F-actin polymerization with latrunculin-A suppressed the pronounced accumulation of CaM at growing germ tube and CAT tips. The movement of CaM associated with spindle pole bodies was prevented by treatment with the microtubule polymerization inhibitor benomyl. The absence of myo-5 resulted in reduced CAT fusion and the lack recruitment of CaM at growing tips indicating a role for the motor protein, myosin-5, in these processes. Finally, by expressing the genetically encoded Ca2+ sensor GCaMP6s under the control of tef-1 promoter in N. crassa, I have been able to image [Ca2+]c dynamics in this fungus for the first time. Using this I have been able to detect localized [Ca2+]c spikes and waves in conidia, germ tubes and CATs. However, I obtained no clear evidence for localized [Ca2+]c changes being associated with CAT chemotropism or fusion.