Mémoire numérisé par la Direction des bibliothèques de l'Université de Montréal.

<p>Species invasions are more prevalent than ever before. While the addition of a species can dramatically change critical ecosystem processes, factors that mediate the direction and magnitude of those impacts have received less attention. A better understanding of the factors that mediate invasion impacts on ecosystem functioning is needed in order to target which exotic species will be most harmful and which systems are most vulnerable. The role of invasion on nitrogen (N) cycling is particularly important since N cycling controls ecosystem services that provision human health, e.g. nutrient retention and water quality.</p><p>We conducted a meta-analysis and in-depth studies focused on the invasive grass species, Microstegium vimineum, to better understand how (i) plant characteristics, (ii) invader abundance and neighbor identity, and (iii) environmental conditions mediate the impacts of invasion on N pools and fluxes. The results of our global meta-analysis support the concept that invasive species and reference community traits such as leaf %N and leaf C:N are useful for understanding invasion impacts on soil N cycling, but that trait dissimilarities between invaded and reference communities are most informative. Regarding the in-depth studies of Microstegium, we did not find evidence to suggest that invasion increases net nitrification as other studies have shown. Instead, we found that an interaction between its abundance and the neighboring plant identify were important for determining soil nitrate concentrations and net nitrification rates in the greenhouse. In field, we found that variability in environmental conditions mediated the impact of Microstegium invasion on soil N pools and fluxes, primarily net ammonification, between sites through direct, indirect, and interactive pathways. Notably, we detected a scenario in which forest openness has a negative direct effect and indirect positive effect on ammonification in sites with high soil moisture and organic matter. Collectively, our findings suggest that dissimilarity in plant community traits, neighbor identity, and environmental conditions can be important drivers of invasion impacts on ecosystem N cycling and should be considered when evaluating the ecosystem impacts of invasive species across heterogeneous landscapes.</p> / Dissertation

Mémoire numérisé par la Direction des bibliothèques de l'Université de Montréal.

T.gondii is a model organism of the phylum Apicomplexa that infects one third of the human population. While the majority of infections are asymptomatic or manifest with mild flu-like symptoms, toxoplasmosis can be fatal in immunocompromised individuals and in the developing fetus. The lytic cycle of tachyzoite-stage parasites causes damage to the host by repeated rounds of host cell invasion, intracellular replication and lysis of the host cell upon egress. Invasion is a key step for the parasite to maintain its intracellular lifestyle. Apical Membrane Antigen 1 (AMA1) is an adhesin released from a unique set of secretory organelles called micronemes. AMA1 plays a central role in the initial stages of host cell invasion. Although parasites without AMA1 are viable in culture, virulence in an animal model of infection is completely attenuated, highlighting AMA1's functional importance. AMA1 is a type I transmembrane protein with a large ectodomain and a short cytoplasmic tail. The ectodomain of AMA1 interacts with domain 3 (D3) of rhoptry neck protein 2 (RON2), which in turn complexes with RONs 4, 5, and 8 in the host cell. Together, this complex of proteins forms the moving junction, through which the parasite pushes itself during invasion. Rhomboid proteases on the parasite surface cleave AMA1 within its transmembrane domain and parasites expressing a non-cleavable form of AMA1 show reduced invasion of host cells and a growth defect. While much is known about the ectodomain of T. gondii AMA1 (TgAMA1), the fate of the TgAMA1 cytoplasmic tail after cleavage remains unclear, its interacting partners remain unidentified, and its role in invasion or thereafter remains a mystery. To address these questions, we: (a) explored the consequences of TgAMA1-TgRON2 interaction during invasion and (b) generated allelic replacement (AR) parasites with point mutations across the tail of TgAMA1 to determine the effect of these mutations on the parasite's ability to invade host cells. Quantitative proteomic techniques were used to analyze the proteins that bind to the tail of TgAMA1 under these different experimental conditions. The results from this work highlight the importance of TgAMA1 post-translational modifications, and potentially TgAMA1-binding proteins, in regulating invasion-related processes in T. gondii.

Mémoire numérisé par la Division de la gestion de documents et des archives de l'Université de Montréal.

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Cancer cell invasion and metastasis are hallmarks of cancer. It is becoming apparent that the interaction between cancer cells and the surrounding microenvironment are involved in their ability to invade and metastasise. In general, cancer cells can either migrate individually, in an amoeboid or mesenchymal manner, or collectively. The first aim of this thesis was to analyse the role of NG2 in amoeboid to mesenchymal transition (AMT) and Rho/ROCK signalling. We found that NG2 promotes an amoeboid morphology, and increased invasiveness, in a Rho-dependent manner. Secondly, we analysed the role of the major tumour microenvironment (TME) component, cancer-associated fibroblasts (CAFs), on melanoma cell invasiveness. We found the CAF interaction with melanoma cells leads to increased levels of interleukin-6 (IL-6) and IL-8, and this leads to increased invasiveness. Simultaneous blocking of IL-6 and IL-8, using neutralising antibodies, inhibits CAF-dependent invasion. Further analysis of another major component in the melanoma TME, keratinocytes, has highlighted the importance of the tumour cell niche in invasion. Our results indicate that cancer cells have the ability to change morphology, and that the TME plays an important role in melanoma cell invasiveness. Metastatic melanoma treatment has proven...

Previous multi-strain mathematical models have elucidated that the degree of cross-protective responses between similar strains, acting as a form of immune selection, generates different behavioural states of the pathogen population. This thesis explores these multi-strain dynamic states, to examine their robustness and stability in the face of pathogenic intrinsic phenotypic variation, and the extrinsic force of immune selection. This is achieved in two main ways: Chapter 2 introduces phenotypic variation in pathogen transmissibility, testing the robustness of a stable pathogen population to the emergence of an introduced strain of higher transmission potential; and Chapter 3 introduces a new model with a possibility of immunity to both strain-specific and cross-strain (conserved) determinants, to investigate how heterogeneity in the specificity of a host immune response alters the pathogen population structure. A final investigation in Chapter 4 develops a method of reverse-pattern oriented modelling using a machine learning algorithm to determine which intrinsic properties of the pathogen, and their combinations, lead to particular disease-like population patterns. This research offers novel techniques to complement previous and ongoing work on multi-strain modelling, with direct applications to a range of infectious agents such as Plasmodium falciparum, influenza A, and rotavirus, but also with a wider potential for other multi-strain systems.

The human immune-regulatory protein, complement receptor type 1 (CR1, CD35), occurs on erythrocytes where it serves as the immune adherence receptor. It interacts with C3b, C4b, C1q and mannan-binding lectin (MBL). It additionally binds the Plasmodium falciparum protein, Rh4, in the non-sialic acid-dependent erythrocye-invasion pathway, and is also important for rosetting, via an interaction with P. falciparum erythrocyte membrane protein 1 (PfEMP1). A C3b/C4b, and PfEMP1 binding site lies in CCP modules 15-17 (out of 30 in CR1), while polymorphisms that afford advantage to some populations in dealing with severe malaria occur in CCPs 24-25, begging the question central to this thesis – do these polymorphism modulate function, and if so how? We hypothesized that the CR1 architecture apposes CCPs 15-17 and CCPs 24-25 using the exceptionally long linker between CCPs 21 and 22 as a hinge, thus polymorphic variants in CCPs 24-25 modulate functionality in CCPs 15-17. To test this, a panel of recombinant CR1 protein fragments (CCPs 21, 21-22, 20-23, 15-17, 17, 10-11, 17-25, 15-25 and 24-25) were produced in Pichia pastoris along with polymorphic forms of the relevant constructs. After purification, biophysical and biological methods were used to assess whether the linker does indeed act as a hinge, and the comparative abilities of the CCPs 15-25 variants (along with soluble CR1 (sCR1), CCPs 1-3 and the panel of CR1 fragments) to interact with a range of ligands were measured. We found no evidence from NMR for face-to-face contacts between CCPs 21 and 22 that would be consistent with the long linker permitting a 180-degree bend between them. Indeed, based on scattering and analytical ultracentrifugation data, CCPs 20-23 form an extended rather than a bent-back structure. All of the four Knops blood-group variants of the CCPs 15-25 proteins produced similar results according to dynamic light scattering and AUC indicating no structural difference or change in self-association state between variants. In addition, based on the data collected from surface plasmon resonance (SPR), ELISA and fluid-phase cofactor (for factor I) assays, there were no evidence of any difference between the polymorphic forms with respect to their interactions with C3b, C4b, C1q and MBL. Only weak interaction was observed for sCR1, and all CCPs 15-25 variants, with the relevant part of PfEMP1, and there was no measurable difference amongst the variants in disrupting rosettes. The sCR1-Rh4.9 interaction was confirmed by SPR; affinities measured between the binding domain of Rh4 and the panel of CR1 fragments identified CCPs 1-3 (site 1) as the main interaction site. It seemed unlikely therefore that CCPs 24 and 25 could modulate Rh4 binding; indeed none of the four CR1 15-25 variants bound Rh4.9 appreciably. Thus we concluded that allotypic variations in CCPs 24-25 have no measurable effect on the architecture as well as binding of CR1 to its host or parasite ligands The inferred selective pressure acting on these variants likely arise from some other (i.e. besides malaria) geographically localised infectious diseases.