O'Regan, Grace Caitríona; (2020) The role of Huntingtin in innate immune cells. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Huntington's disease (HD) is a devastating neurodegenerative disease. It is caused by the presence of an expanded trinucleotide CAG repeat in the HTT gene, resulting in expression of expanded polyglutamine-containing mutant HTT throughout the body. HD patients display a dysfunctional peripheral immune system up to sixteen years before disease onset. This is shown also in HD animal models, where immune system dysfunction is found both in the periphery and central nervous system (CNS). The overall aim of this work is to determine the mechanism(s) by which mutant and wild type HTT regulate myeloid cell function and contribute to innate immune system dysregulation as a potential modifier of HD progression. Specifically, this thesis had two aims: firstly, to investigate the CNS component of the innate immune system in a human HD microglia-like cell model; and secondly, to assess the role of wild type HTT in peripheral innate immune cells. To address the first aim, a unique cohort of human induced pluripotent stem cell lines from related individuals with varying HTT polyglutamine expansion lengths (22Q, 58Q, 69Q, 75Q) and an isogenic series of embryonic stem cell lines (30Q, 45Q and 81Q) were differentiated to microglia-like cells. Our results suggest that these HD microglia-like cells display subtle altered phenotypes compared to control microglia in a HTT polyglutamine length-dependent manner. To investigate the impact of these altered phenotypes on HD pathology, a series of conditioned media experiments were conducted to assess the effects of HD or control microglia-like cell-conditioned media from basal and stressed conditions on HD and control medium spiny neurons, and vice versa. Finally, to assess the performance of HD and control microglia-like cells in an environment closer to what would be found in the CNS, microglia-like cells were treated with CSF from HD and control individuals and their health and function assessed. This work represents the first assessment of human HD microglia in vitro and shows that human HD microglia-like cells are dysfunctional like their rodent counterparts and peripheral myeloid cells from HD patients. For the second aim, glucan-encapsulated siRNA particles were used to lower HTT levels in non-HD primary human macrophages and a series of functional assessments were performed, comparing HTT-lowered and control cells. These findings suggest a novel role for wild type HTT in the myeloid cells of the peripheral immune system, specifically in cytokine expression, phagocytosis and their response to cellular stress. Together, these results further elucidate the role of huntingtin in innate immune cells in health and disease.

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