Multi-nucleated glial cells and the implication for neural health

Abstract

The term glioma encompasses all tumours that originate from glial cells and accounts for almost 80% of primary malignant brain tumours. Gliomas typically present as a heterogeneous mass of cells that include the presence of multi-nucleated cells. Infectious agents are a significant risk factor in gliomagenesis and brain cancer, particularly those agents that are known to cause multi-nucleation of cells. To date, the infectious agents that have been implicated in gliomagenesis include viruses, protozoans, and possibly Brucella species. However, there is currently no evidence of bacteria directly causing multi-nucleation of glia resulting in gliomagenesis. The bacteria Burkholderia pseudomallei and Neisseria meningitidis, the causative agents of melioidosis and meningitis respectively can penetrate the central nervous system (CNS) via infection of the olfactory and trigeminal nerves. Importantly, the penetration of the CNS by B. pseudomallei and N. meningitidis can occur at very low bacterial levels such that CNS infection can be asymptomatic, and the bacteria can persist within the body for many years or even decades. The effect of long-term presence of subclinical levels of the bacteria within the CNS is unknown and B. pseudomallei and N. meningitidis infections have not yet been implicated in gliomagenesis. The in vitro work in our laboratory has determined that B. pseudomallei rapidly initiates the formation of multinucleated glial cells, including olfactory ensheathing cells (OECs), Schwann cells and astrocytes. These results suggest that the bacteria may initiate a cascade of events leading to multi-nucleation of glial cells; as multi-nucleated cells are a pathology that is a characteristic of glioma it raises the question of whether the glia cells exhibit other characteristics associated with inflammation and/or glioma. The goal of this thesis is to characterise the cellular and molecular events resulting from bacterial initiation of multi-nucleated glial cells. The project consists of three aims. The first aim is the study of multinucleated cell (MNC) formation in Schwann cells after infection with B. pseudomallei and N. meningitidis serogroup B in vitro. The second aim is to determine the changes in the expression of molecules associated with gliomagenesis in multinucleated glial cells. The third aim is to study the energy production in Schwann cells following the infection with B. pseudomallei. Trigeminal Schwann cells were infected with B. pseudomallei and N. meningitidis serogroup B in various ratios and formats and examined for formation of MNCs. The immunofluorescence microscopy results showed that Schwann cells formed MNCs following the infection with both bacteria. The results also demonstrated that infection with both bacteria is associated with the expression of proteins and markers associated with inflammation and glioma. Immunoblotting, proteomics and qRT-PCR results showed that the expression of inflammatory and gliomagenesis markers were upregulated significantly after the infection with B. pseudomallei and N. meningitidis serogroup B. These results suggest that the bacterial infection of glia leads to responses consistent with inflammation and/or initiation of gliomagenesis. We also demonstrated that B. pseudomallei infection can mis-regulate the expression of Warburg effect genes in favour of aerobic glycolysis energy production which is a characteristic feature of carcinogenesis. In conclusion, in this study we showed that infection of glia with facultative intracellular bacteria such as B. pseudomallei and N. meningitidis results in molecular and cellular changes consistent with inflammatory responses and potentially associated with glioma.