Abstract
There are converging lines of the evidence that microglia, innate myeloid cells of the central nervous system, might be implicated in psychiatric disorders with a putative neurodevelopmental origin such as schizophrenia and autism spectrum conditions (ASCs). The complex genetic and environmental aetiology of these disorders and heterogeneous clinical presentations have restricted translational potential of disease modelling with animals. Human postmortem studies exist but represent microglia phenotypes at end of life rather than during development, and studies using induced microglia models from blood monocytes is limited by not recapitulating authentic microglia ontogeny while also representing post-diagnosis phenotypes. Alternative human in vitro models are needed, with human fetal primary tissue being limited and not ethically suitable. Therefore, this thesis reviewed evidence for microglia involvement in schizophrenia and ASC and proposes that human induced pluripotent stem cells (hiPSCs) could be a useful model due the potential of generating microglia-like cells (MGLs) with authentic microglia ontogeny. To study the potential contribution of microglia in the development of schizophrenia and ASC, I characterised hiPSC-derived MGLs from donors with a highly penetrant risk factor risk factor for both conditions, 22q11.2 deletion syndrome (22q11.2DS), which is caused by one or more hemizygous microdeletions in the 22q11.2 chromosome region.Genotyping of hiPSCs from donors with 22q11.2DS was performed, identifying several deleted genes expressed in microglia. Polygenic risk was calculated, highlighting potential sources of variance among 22q11.2DS donors. Following confirmation that hiPSCs from donors with 22q11.2DS can successfully differentiate to MGLs, this thesis showed that 22q11.2DS affects MGL form and function. Compared to hiPSC-derived MGLs from neurotypical donors, 22q11.DS MGLs have reduced arborisation, higher TMEM119:IBA1 colocalisation, increased phagocytosis, a transcriptomic profile suggesting differences in cytokine signalling, and reduced IL-6 and IFNγ cytokine secretion.
Differences between 22q11.2DS MGLs and control MGLs also appears to be context-dependent. Transcriptomic analysis following lipopolysaccharide (LPS) challenge identified differences in pathways associations and differentially expressed genes, with interaction analysis suggesting a stronger effect of LPS on the expression of CXCL10 and CHN1 in 22q11.2DS MGLs. Additionally, LPS challenge induced higher phagocytic activity, inferred lysosome levels using lysotracker, and secretion of IL-10 and TNFα alongside reduction in TMEM119:IBA1 colocalisation in control MGLs compared to 22q11.2DS MGLs. On the other hand, LPS challenge did not impact 22q11.2DS MGLs morphological state even though it reduced polarity in control LPS MGLs but induced enhanced IL-6 secretion compared to control MGLs.
This thesis not only provides the first characterisation of microglia with 22q11.2DS, but also is the first hiPSC study investigating microglia contribution to schizophrenia and ASC pathophysiology. The context-dependent impact of 22q11.2DS on MGLs highlights key mechanisms which could contribute to the development of schizophrenia and ASC. Findings both overlapped and contrasted with previous evidence from in vitro and in vivo human and animal models, including data on human induced microglia from blood monocytes. This suggests that models with authentic microglial ontogeny may be important in studying the contribution of microglia to the development of 22q11.2DS-associated schizophrenia and ASC. Increased replication with stratification of donors by diagnosis and polygenic risk, comparison to idiopathic donors, and use of isogenic lines is recommended. Subsequently, characterisation should be extended to more complex model system like co-cultures and organoids to assess microglial interaction with other cell types including neurons, and in the future also to chimeric animals to examine impact on behavioural phenotypes.
| Date of Award | 1 Nov 2023 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Anthony Vernon (Supervisor) & Deepak Srivastava (Supervisor) |