Zebrafish disease modelling identifies a link between type I interferon signalling and cholesterol dysregulation in Aicardi-Goutieres syndrome and brain endothelial cells

Abstract

Aicardi Goutieres Syndrome (AGS1-9) is a rare inflammatory mediated encephalopathy which occurs due to mutations in genes involved in the sensing, regulation and metabolism of self-nucleic acid species. Proteins implicated in AGS play an essential role in self-non-self-discrimination, which helps explain the excessive type I interferon (IFN) production and large upregulation of interferon stimulated genes (ISGs) that occurs in AGS patients. This strong antiviral response is thought to largely contribute to the devastating nature of the condition, which results in significant physical and neurological disability. AGS patients can also suffer from sub-type specific symptoms, such as cerebrovascular disease which only manifests in patients with mutations in SAMHD1 (AGS5). Indeed, AGS5 represents a monogenic form of inflammatory based stroke, and can be utilised to provide unique insight into the mechanisms and causes behind more common forms of stroke, such as viral infections as a risk factor, whereby excessive type I IFN signalling as an antiviral mechanism may be related to the onset of stroke. Moreover, antiviral signalling has also been attributed to cholesterol dysregulation, an additional clinical risk factor for both ischaemic and haemorrhagic stroke. To date, few mouse models successfully replicate the vast clinical phenotypes observed in AGS patients, with none phenocopying the neurological manifestations which are one of the hallmarks of the condition. Contrastingly, transient knockdown of the samhd1 gene in a zebrafish larval model appeared to recapitulate a number of clinically relevant phenotypes, including neurological aspects of the disease, as observed by the presence of intracerebral haemorrhage (ICH) in the larvae. Therefore, zebrafish may offer specific advantages over mice when modelling AGS pre-clinically. As such, the central aim of this PhD was to characterise a novel stable mutant zebrafish model of AGS5 to increase understanding of SAMHD1-induced AGS, and the associated cerebrovascular disease. Zebrafish disease modelling of AGS5 revealed the presence of a number of neurological phenotypes, not previously identified in the existing pre-clinical models, reinforcing the usefulness of zebrafish as a model organism to study AGS. Furthermore, the zebrafish exhibited cerebrovascular deficits and dysregulation of the cholesterol biosynthesis pathway, which was also observed in an AGS patient RNAseq dataset. Further investigation of the type I IFN signalling relationship with cholesterol in human brain endothelial cells, identified cholesterol dysregulation following treatment of the oxysterol 25 hydroxycholesterol (25HC), produced downstream of type I IFN. A reduction in cholesterol from the plasma membrane acted to alter the normal functioning properties of the brain endothelial cells. Taken together, we propose that these data in zebrafish and human cells provide new mechanistic insight into how cholesterol dysregulation, as a result of antiviral signalling, may lead to the generation of cerebrovascular deficits and stroke.

Date of Award	27 May 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Stuart Allan (Supervisor), Paul Kasher (Supervisor) & Tracy Briggs (Supervisor)