title: Targeting the type 5 metabotropic glutamate receptor in a mouse model of terminal neurodegeneration
Creators: name: Budgett, Rebecca
Publisher: name: University of Glasgow
Year: 2024

description: The type 5 metabotropic glutamate receptor (mGlu5) plays an important role in learning and memory processes and has been identified as a potential drug target for the treatment of neurodegenerative diseases (NDD). Given the limited treatment options available to patients living with NDD, there is a pressing need for novel therapeutic interventions that treat both the symptomatic and progressive components of neurodegeneration. Promisingly, the pharmacological and genetic blockade of mGlu5 has been shown to reduce disease pathology and improve cognition in several preclinical mouse models of neurodegeneration (Budgett et al., 2022). Expressed in both neurons and glia, mGlu5 has been shown to play a role in neuroinflammation and, therefore, represents a potential target for regulating neuroinflammation in disease (Byrnes et al., 2009). Therefore, this thesis aimed to evaluate the role of mGlu5 in the modulation of neuroinflammation and the progression of NDD in a model of terminal neurodegeneration, murine prion disease. 

To provide insights into the signalling mechanisms of mGlu5 and the characteristics of novel compounds, in vitro functional assays were conducted on cell lines expressing mGlu5. Flp-in cells expressing mouse mGlu5 and mouse primary cortical astrocytes were found to be suitable systems for investigating the signalling properties of mGlu5 ligands. In these systems, VU0424238 was confirmed to be a mGlu5 negative allosteric modulator (NAM) that binds to the main allosteric binding site on mGlu5 (Felts et al., 2017). Furthermore, target engagement and efficacy in vivo were established. 

Recent studies have shown close correlates between murine prion disease and human NDDs, including hippocampal-based cognitive deficits, neuroinflammation, and terminal neurodegeneration (Bourgognon et al., 2018; Bradley et al., 2016). Here, the model of murine prion disease was investigated using histological and biochemical studies to characterise the neuroinflammatory response throughout disease progression. The astrocytic and microglial markers GFAP, vimentin, Iba-1, and CD68 were confirmed to be upregulated in murine prion disease, in addition to several pro-inflammatory cytokines. This is similar to human NDDs characterised by chronic neuroinflammation.

Subsequently, this thesis aimed to define the impact of mGlu5 blockade, both pharmacologically and genetically, on the progression of murine prion disease. Firstly, mGlu5 was inhibited pharmacologically using the NAM VU0424238. Although chronic VU0424238 treatment significantly reduced the expression of GFAP in the hippocampus of female prion-diseased mice, mGlu5 antagonism did not alter the overall progression of murine prion disease. There were no alterations to the accumulation of misfolded prion protein, symptom onset, survival, or behaviour after chronic VU0424238 treatment. Next, mGlu5-deficient mice were inoculated with prion disease. Although mGlu5 deficiency resulted in the reduced expression of the inflammatory markers Iba-1 and GFAP in early-stage disease, it did not affect overall disease progression. 

Overall, these findings suggest that mGlu5 may represent a potential approach by which neuroinflammatory processes in NDDs might be modulated. However, the results also suggest that mGlu5 may not play a significant role in the progression of prion disease and may function differently in prion disease compared to other NDDs.