Hybrid Brain-Tissue Based Model to Study the Role of Infiltrating Monocytes in Late Stage Huntington's Disease

Huntington’s Disease (HD) is an inherited, progressive, fatal, and late-onset neurodegenerative disorder. Its neuropathogenesis is characterized by selective loss of medium spiny neurons (MSNs) in the striatum and cortical neurons in the cortex. It is caused by an expansion of CAG repeats in the exon-1 of IT-15 gene, which results in a mutated polyglutamine domain in the huntingtin protein (HTT) (Collaborative, 1993). The mutation that causes HD was identified almost thirty years ago, yet the ultimate cause of neuron death is still uncertain. This dissertation is based on the hypothesis that monocytes infiltrate the brain of late stage HD patients and drive neuropathogenesis. To test this hypothesis, I developed a novel hybrid brain-slice experimental model in which human monocytes (MO) isolated from HD and non-disease control patients (CTRL) are engrafted into living postnatal striatal brain tissues prepared from neonatal rats.

In the first set of experiments, the inflammatory response of stimulated HD and CTRL MO were tested. HD MO exhibited an increased inflammatory response demonstrated by increased production of cytokine IL-10. Furthermore, a late-stage HD patient exhibited an increased production of multiple cytokines, including IL-10.

The second set of experiments utilized a brain-tissue based model to measure the impact of engrafting HD MO on MSN health. In brain slices engraftment with HD MO, there was a decrease in the number of healthy MSNs. A further experiment revealed that stimulating MO prior to their engraftment differentially influenced the survival of MSNs depending on whether the MO came from HD or CTRL patients. Measurement of cytokine production from these unstimulated/stimulated engraftment experiments revealed a trend for increased cytokine production in samples taken from a moderate stage HD patient.

The third set of experiments assessed the ability of HD MO to influence the endogenous macrophages within the brain slice tissue, microglia. MO engraftment, independent of its genotype, altered microglia morphology within brain slices. Further results from these experiments demonstrated that the presence of HD MO increased microglial density and upregulated their phagocytosis of MSNs. While the mechanisms underlying these multicellular interactions remain to be determined, it is possible that such an increase in phagocytosis could lead to the observation of fewer healthy MSNs as quantified in the second set of experiments.

Together, these three sets of experiments support the application of a novel model to study neuropathogenesis and highlight that the genotype of infiltrating MO has the potential to quantifiably alter intercellular interactions and neuronal health in the context of HD.