Investigating the role of Casd1 in the host response to malaria infection
Creators: name: Ilyas, Sadaf
Publisher: name: The Australian National University
Year: 2021

description: Malaria is a global health concern responsible for thousands of deaths worldwide annually. Concerted efforts employing various preventive and treatment strategies have contributed a lot in controlling the disease, but we are still far from achieving complete eradication. Antimalarial drugs have been on the forefront of malaria control strategies, but their effectiveness is jeopardised due to increasing development of drug resistance in parasites. Interestingly, it has been noted that certain genetic mutations, especially those in red blood cells, confer natural resistance against malaria in endemic populations. Screening such mutations have promising translational potential as they may lead towards novel therapeutic targets or host-directed-therapies to overcome malaria resistance. In this study, N-Ethyl-N-Nitrosourea (ENU) mutagenesis, has been used as a tool to screen for novel erythrocyte variants in mouse model and assess their role in malaria susceptibility. A recent phenotypic screen for red cell abnormality identified a novel Casd1 gene mutation in mice that conferred a complete protective effect against P. chabaudi and P. berghei rodent malaria parasite. Casd1 is a eukaryotic 7(9)-O-acetyltransferase that resides in the lumen of Golgi complex as a membrane integrated protein. It functions by acetylating sialic acid residues that are subsequently decorated on the cell surface. The mutation was characterised to be a single base substitution at the splice site of Casd1 gene leading to skipping of the exon 14. An in silico analysis of the secondary structure of mutant Casd1 protein predicted a loss of connecting loop between two transmembrane helices affecting the topology of transmembrane domain. The initial screen identified a loss of Ter119 binding to erythrocytes derived from Casd1 mutated mice (ENU25^Casd1/Casd1). Ter119 is an erythroid lineage marker that recognises 7-9-di-O acetyl sialic acid epitope on the erythrocyte surface. Abrogation of Ter119 signal from the erythrocyte surface may indicate the loss of Casd1 function in ENU25^Casd1/Casd1 mice. The erythrocytes from ENU25^Casd1/Casd1 mice were further assessed for their susceptibility to invasion by malaria parasites. The relative parasitaemia was significantly reduced in the mutant erythrocytes as compared to the wild type, suggesting a novel role of Casd1 in merozoite invasion. Furthermore, the parasitised erythrocytes were tracked during infection assays to assess the role of Casd1 mutation in clearance of malaria parasites. These assays revealed a drastic clearance of exogenous blood injected into ENU25^Casd1/Casd1 mice as compared to wildtype blood. The clearance response in ENU25^Casd1/Casd1 mice was subsequently identified to be dose dependent and non-specific to malaria infection. Overall, these findings suggest that Casd1 plays a role in malaria susceptibility by modulating merozoite invasion. The results also revealed a novel role of Casd1 in self-nonself recognition which results in bystander clearance of the malaria infected foreign red blood cells in ENU25^Casd1/Casd1 mice.