Toxoplasma gondii is a parasite that chronically infects the central nervous system of over a third of the global populations. Characterised by resistance to immune and pharmaceutical clearance, parasite cysts serve as a reservoir for reactivation in the face of immune compromise. Development of new and more effective therapeutics against this stage of infection is hampered by the lack of insight into the biology of chronic infection.
In an effort to avoid clearance, many pathogenic microorganisms secrete effector proteins that modulate host signalling and immunity. While effector protein export is well-studied in acute toxoplasmosis, its contribution to the establishment and persistence of chronic infection remains unclear.
During her PhD, Ushma has built on previous studies to investigate the contribution of effector protein export to the long-term persistence of T. gondii infection. She has also explored the role of host cell death pathways involved in the resolution of acute infection and how protein export may be affecting these pathways. Using mouse models of acute and chronic toxoplasmosis, her work provides evidence for novel interactions between host defence mechanisms and how this parasite attempts to subvert them in order to avoid clearance. Understanding the intricacies of these host-pathogen relationships will facilitate development of better therapeutics to ultimately reduce burden of disease.
The Tumor Necrosis Factor (TNF) is the eponymous member of the TNF superfamily, which plays diverse roles in cell death, proliferation, inflammation and the immune response. It predominantly signals via binding to TNFR1, and is able to induce very distinct outcomes, transcriptional as well as cell death response. Dysfunction of TNFR1 signalling can cause several diseases, hence TNFR1 signalling is tightly regulated. Despite the growing appreciation of how TNFR1 downstream signalling events are propagated and how cell survival and cell death decisions are made, we still do not have a clear picture of how is TNFR1 signalling activated by ligand binding to the receptor, and how it is monitored after activation.
My PhD research has focused on the influence of TNFR1 oligomeric status on receptor activation. As TNF ligand often exists in trimeric form, it is assumed trimeric ligands induce the trimerization of the receptor, thereby initiating signalling. However, I found changing the oligomeric status of TNFR1 through its transmembrane domain (TMD) does not affect TNF-induced signalling activation, and TNFR1 variant bearing monomeric TMD causes receptor auto-activation. Combing the effects of various TNFR1 variants bearing extracellular domain (ECD), TMD, and/or death domain (DD) mutations, it suggests that ECD and TMD play a role in inhibiting TNFR1 auto-activation in the absence of ligand, and monomeric TMD is less efficient in this inhibitory function.
After TNFR1 activation by TNF ligand, it is believed that ubiquitination event of kinase RIPK1 plays crucial role in determining cellular outcomes. RIPK1 is also known to be the master regulator of cell death, and can induce both apoptosis and necroptosis. During the apoptosis, RIPK1 is cleaved by caspase 8, and this prevents the formation of necrosome. However, what happens next to RIPK1 after cleavage becomes obscure. Whether the cleavage products are functional or are directed to the rapid turnover is unknown. Some research suggests that the C-terminal fragment of RIPK1 is pro-apoptotic, but there is no direct evidence. In my PhD research, I found the RIPK1 C-terminal fragment is targeted by cysteamine dioxygenase (ADO) and consequently is subjected to N-degron degradation. The regulation though N-degron pathway is also affected by oxygen level. I aim to study the potential role of RIPK1 C-terminal fragment in cell death and how it is regulated by N-degron pathway.