Deciphering the impact of lipopolysaccharide and high-fat diet on disease pathogenesis in Parkinson’s and Huntington’s disease rat models

Abstract

Neurodegenerative diseases such as Alzheimer’s, Parkinson’s, and Huntington’s disease affect millions of people worldwide, with no effective treatments currently. They are often characterised by the accumulation of misfolded proteins, such as tau, amyloid beta, alpha-synuclein, and huntingtin (Sweeney et al. 2017). Aging, genetic, and environmental factors play a significant role in the progression of these diseases. Thus, a detailed understanding of these diseases would shed light on the development of therapeutic targets. Parkinson’s and Huntington’s diseases are associated with neuronal loss in the motor and sensory systems (Troncoso-Escudero et al. 2020). A combination of genetic and environmental factors contributes to Parkinson’s disease, with only 10-15% of cases resulting from genetic mutations, while the rest are sporadic. (Ball et al. 2019). Many environmental factors, including pesticides and heavy metals, have been linked to the onset of Parkinson’s disease. Neuropathologically, Parkinson’s disease is characterized by the presence of Lewy bodies, which are composed of aggregates of alpha-synuclein (Wakabayashi et al. 2007). In comparison to Parkinson’s, Huntington’s disease is an autosomal dominantly inherited disorder caused by a trinucleotide repeat expansion (CAG) within the gene Huntingtin HTT (Nopoulos 2016). CAG codes for the amino acid glutamine, and the expansion of polyglutamine tracts results in the formation of neuronal aggregates containing the mutant HTT (Kshirsagar et al. 2021). For the pathophysiology of neurodegenerative diseases, the role of misfolded proteins leading to neurodegeneration has been discussed earlier. Recently, substantial evidence suggests that neuroinflammation plays a role in the pathogenesis of these diseases (Kwon and Koh 2020). Neuroinflammation can be triggered by environmental pollutants, including metals, diet, pollution and exposure to pesticides (Langley et al. 2022). It is crucial to understand how neuroinflammatory factors can trigger neurodegeneration and whether environmental factors play a role as risk factors in these diseases. Previous studies have demonstrated that inflammatory stimuli can induce long-term epigenetic memory in microglia, thereby significantly influencing the deposition of amyloid beta plaques in the brain of an Alzheimer’s mouse model (Wendeln et al. 2018). Studies have also shown that neuroinflammation plays a significant role in the pathogenesis of Parkinson’s and Huntington’s diseases (W. Zhang et al. 2023). However, the role of microglial immune memory in influencing pathogenesis in Parkinson’s and Huntington’s disease remains unknown. This thesis aims to investigate how immune memory is triggered with bacterial lipopolysaccharide, LPS, and high-fat diet, HFD, and whether this immune memory shapes neuropathology in Parkinson’s and Huntington’s disease. Here, we use the BAC SNCA and BAC HD rat models, which were generated using a bacterial artificial chromosome (BAC) containing the entire human SNCA or HTT genomic sequence with 97 polyQ repeats and all regulatory elements (Nuber et al. 2013; Yu-Taeger et al. 2012). In summary, we report on changes in body weight of the BAC SNCA and BAC HD rats when exposed to bacterial lipopolysaccharides and a high-fat diet. LPS-induced sickness behavior in the rats led to temporary weight loss after the injections. In contrast, HFD rats gained weight within four weeks compared to the control group. The changes in body weight were compared with different cytokine responses and indicated that after the first LPS injection (1xLPS), an increase in the pro-inflammatory cytokines (IFN-γ, IL-1β, TNF-α) was observed, but repeated injections of LPS (2xLPS and 4xLPS) resulted in a suppression of the cytokine levels (P. Liu 2022). Several post-translational modifications are associated with alpha-synuclein, and patients with Parkinson’s disease have alphasynuclein phosphorylated at Serine 129 (Kawahata, Finkelstein and Fukunaga 2022). This is typically found in the neuronal cells of patients and is more pronounced in the late stages of the disease. Additionally, alpha-synuclein phosphorylation is widespread in Lewy body pathology (Takahashi et al. 2002). At the histological level, we observed the presence of phosphorylated alpha-synuclein in the forebrain of the BAC SNCA rat model at the age of 9 months. Upon quantification of the signal, an increase in the area of phosphorylated serine 129 deposits could be observed with HFD and LPS treatments. In the BAC HD rat model, histological nuclear localization of the mutant huntingtin could be observed following LPS treatment. Accordingly, we emphasized that HFD showed opposite effects at the level of insoluble protein detection in BAC SNCA and BAC HD rat models. On one hand, HFD leads to an increase in insoluble alpha-synuclein protein aggregates. On the other hand, it reduces the accumulation of insoluble huntingtin in the forebrain at the age of 9 months. Moreover, we also detected soluble-level alpha-synuclein and huntingtin in both models, and an expression pattern of full-length and fragmented alpha-synuclein could be observed. Different forms of alpha-synuclein, including monomeric, dimeric, trimeric, tetrameric, and oligomeric forms, could be detected. HFD led to an increase in the oligomeric forms of alpha-synuclein. Although the detection of huntingtin soluble protein revealed the presence of the full-length huntingtin protein along with the fragmented portion, no treatment-based effects of LPS and HFD could be observed in our BAC HD rat model. Furthermore, bulk RNA sequencing of the 9-month-old BAC SNCA model revealed oxidative phosphorylation as one of the altered pathways, thereby verifying mitochondrial dysfunction at the OXPHOS protein level in the BAC SNCA rat model. HFD and LPS treatment showed a reduction in the complex V of the mitochondrial complex. Even when there are alterations in the accumulation of alpha-synuclein and huntingtin protein levels, no autophagy response could be observed in the BAC SNCA and BAC HD disease models. Inflammatory markers were used to check the role of HFD and LPS in inducing inflammation in the BAC SNCA model. A reduction in LPS groups with β-arrestin 1 confirms the stimulation of microglial immune responses. Overall, the data in this thesis demonstrate the role of LPS and HFD in triggering immune responses in the brain in BAC SNCA and BAC HD rat models. It also provides the first evidence that LPS and HFD modulate brain pathology responses in these disease models.