Go with your Gut: How inflammation can speed up Motor Dysfunction in Alpha Synuclein Mutant Mice

Sonita Mohammadi

Parkinson's Disease (PD) is a neurodegenerative disorder that causes motor symptoms bradykinesia, rigidity, and tremor (de Lau & Breteler, 2006) PD prevalence increases with age and affects 1% of the population over the age of 60 (de Lau & Breteler, 2006). No cure exists and pharmacological therapies are available to diminish the symptoms of the disease. Recent studies suggest gut microbiota can lead to the formation of alpha-synuclein in the enteric nervous system (ENS) and can travel via the vagus nerve to the central nervous system (CNS) (Fitzgerald et al., 2019). Alpha-synuclein is an important factor of Lewy body’s production that comes from in the loss of dopaminergic neurons in the substantia nigra as a cause of PD (Pickrell et al., 2015). Studies are trying to prevent Lewy body development by focusing on alpha-synuclein aggregates (Pickrell et al., 2015). The initial paper by Kishimoto et al., (2019) explored the role of chronic mild gut inflammation and how it plays a role in hastening the onset of motor dysfunction in PD mice (Kishimoto et al., 2019). In their animal model, Parkinson's disease (PD) mice treated with dextran sodium sulfate (DSS) in their water for 12 weeks and observed that the onset of motor disorder sped up (Kishimoto et al., 2019). The PD mutant DSS- treated mice exhibited motor dysfunction considerably earlier than their control group (Kishimoto et al., 2019). This study concluded that a chronic mild increase in gut inflammation speeds up the onset of motor dysfunction in PD.

Key words: Parkinson’s disease, inflammation, alpha-synuclein, Enteric neurons, Neuroinflammation

INTRODUCTION

Parkinson's Disease (PD) is a neurodegenerative movement disorder that is a multi-etiological condition with uncertain etiopathogenesis (de Lau & Breteler, 2006). Genes and environmental hazards both play a part in the pathology of PD (Chen et al., 2019). PD is an age-related disorder affecting individuals over the age of 6o years of age (Antony et al., 2013). The hallmark of PD is the destruction of dopaminergic neurons in substantia nigra and alpha-synuclein presence in Lewy-bodies (Weil, R. S., Lashley). Losing these neurons results in manifestation of motor symptoms such as bradykinesia, resting tremor, and other nonmotor symptoms like depression, constipation, (Chen et al., 2019). The modern mechanism of PD is that mitochondrial impairment can lead to oxidative damage and that aggregation of alpha-synuclein. These aggregates play a key role in degradation of dopaminergic neurons (Hu & Wang, 2016). In addition, microglial cells produce proinflammatory cytokines with neuronal degradation in PD (Kishimoto et al., 2019). Kishimoto et al., (2019), animal models suggest that alpha-synuclein aggregation may first emerge from the peripheral neurons (Kishimoto et al., al., 2019). DSS-treated mice showed a significant decline in

2019). Kishimoto et al., (2019) propose that alpha-synuclein travels retrogradely from the gut to different regions of the brain (Fitzgerald et al., 2019). They base this idea on the understanding that patients with PD often have a history of chronic constipation and suffer from vagal cholinergic tone deficit (Kishimoto et al., 2019). Earlier studies found that alphasynuclein pathology is present in gut 20 years before they diagnosed individuals with PD (Kishimoto et al., 2019). Therefore, studies shifted their attention to the processes by which the gut inflammation contribute to the production of alpha-synuclein in the enteric nervous system (Fitzgerald, Emily, et al.). The original paper examines whether chronic mild gut inflammation in PD mouse model can speed up motor dysfunction symptoms (Kishimoto et al., 2019). Kishimoto et al. 2010, hypothesized that chronic mild gut inflammation can speed up the onset of PD and increased neuroinflammation in the brain (Kishimoto et al., 2019). They induced inflammation in the gut via dextran sodium sulfate (DSS) in PD mice and wild type. They observed significant motor dysfunction, alpha-synuclein aggregation and degeneration in dopaminergic neurons in the DSS treated PD mice compared to the control group (Kishimoto et al., 2019). To confirm that DSS was the only cause for inflammation, they tested serum cytokines levels in all four groups, and they revealed no significant variations. Next, the authors studied immunoassayed colon and brain tissue with an antibody specific for microglia and macrophages for local tissue inflammation; and discovered induced inflammation in the gut and brain tissue. Their result showed that chronic mild gut inflammation can speed up the onset of PD (Kishimoto et al., 2019). Although the mechanism and physiological functions of alpha-synuclein is not clear (Bernal-Conde et al., 2020), Kishimoto et al., (2019) concluded that their result was consistent with chronic mild gut inflammaminergic neurons by travelling retrogradely via vagus nerve (Kishimoto et al., 2019). Therefore, understanding the cause of complex neurodegenerative disorders is critical for developing effective therapies. Modern therapies for PD offer no cure and mildly ease the motor symptoms. This review will seek to shed light chronic mild gut inflammation and Parkinson’s disease (Kishimoto et al., 2019).

Visual Abstract

Figure 1. Visual abstract to represent the experiment carried out by Kishimoto et al. regarding the role of chronic mild gut inflammation and onset of motor dysfunction in PD (Kishimoto et al., 2019). In this experiment, the authors reported that chronic mild gut inflammation is suffices to speed motor disorder symptoms in PD mice (Kishimoto et al., 2019).

Major Results and Methods

The authors used female transgenic mice, over-expressing alphasynuclein (PD mice) and age-matched wild type (WT) mice in their experiments. They randomly assigned PD and WT mice to different treatments, either treated DSS in their drinking water or just water for 12 weeks and mice euthanized. To determine motor performance, Kimshimo and colleagues used 3 unique motor test tasks, a rotarod apparatus, rotating rod and gate and grip analysis (Kishimoto et al., 2019). The authors observed that mutant PD mice developed discernable motor symptoms and become progressively worse at performing motor test tasks performance until the mice no longer could ambulate (Kishimoto et tion exacerbating alpha synuclein in the brain and destroy dopa

rotarod performance Figure 1 (a), grip strength (b) and stride length (c) (Kishimoto et al., 2019). These data are significant as they showed that there was a considerable difference in disease progression in PD mice induced with compared to the Wilde type mice (Kishimoto et al., 2019). Kimshimo et al. results are consistent with previous findings in literature. Figure 2 results are significant because it showed that chronic mild gut inflammation leads to loss of dopaminergic neurons in alpha-synuclein mutant transgenic mice

Motor Tests Performance result

Figure. 1 a b c Retrieved images from Kishimoto et al., 2019.Figure 1. a The graph shows the result of grip strength test. b The graph shows the result of stride length test. c Image show ink footprints (Kishimoto et al., 2019). These results are significant because PD

mice treated with DSS developed discernable motor dysfunction (Kishimoto et al., 2019).

Chronic gut inflammation exacerbates alpha- thology

synuclein pa

et al., 2018). Although we have seen that chronic mild inflammation in the gut lead to an increase in alpha-synuclein aggregation in the brain and degradation of dopaminergic neurons, these authors did not discuss the exact cellular mechanism. Their research concentrated more on the link between gut inflammation and motor dysfunction. A limitation to this paper is failing to present specific mechanistic evidence in how inflammation in the gut travels via the vagus nerve. Although their result complied in the research study supports their hypothesis, further research required in this developing field.

Future studies

Future studies should involve understanding the alphaplace weekly. At the end of the 8-week study period, rats will be euthanized to examine the brain and the gut tissues. Techbe performed to observe tissue changes and measure alpha

Figure. 2 a b

Retrieved images from Kishimoto et al., 2019. Figure. 2 a) The images show tyrosine hydroxylase (TH) in substantia nigra. b Is a sagittal section from mice showing alpha synuclein and immediately below each image is a high magnification micrograph image of the midbrain. 200um scale bar. The top image shows substantia nigra WT/H2O and PD/H2O (upper) and WT/DSS and PD/DSS are (lower) region.

Discussions Section

Kishimoto et al. (2019) concluded that their findings are important in treatment and prevention of PD by finding a link between chronic inflammation in the gut and neurodegenerative disorders. Alpha-synuclein mutant transgenic PD mice displayed discernable motor deficits, as they could not move at the end of the 12-week study period. Using immunoblot technique, it allowed the author to visualize the increased levels of alpha-synuclein protein and decrease tyrosine hydroxylase dopaminergic neurons compared to the wild type. This allowed them to link the chronic mild gut inflammation to increase in alpha-synuclein aggregation in the brain and degeneration of dopaminergic neurons by a mechanism that aggregates travel retrogradely via the vagus nerve to different regions of the brain. Kishimoto et al. (2019) recognized that their results are consistent with current literature, particularly when explaining that PD may first develop in the peripheral neurons, enteric branches of the vagus nerve. In an experiment conducted in mice by Johns Hopkins Medicine, researchers showed there is reliable evidence that Parkinson’s disease develops in the gut

synuclein proinflammatory role in association to diet. One proposed study can investigate the change in alpha-synuclein expression using a mouse model with and without PD, and the effects of a high-fat diet on motor function. Models will include neurotoxin 6-hydroxy dopamine (6-OHDA) rat to induce PD and transgenic rats that do not exhibit PD. The Rats will randomly be assigned to a normal diet or ketogenic for 8 weeks. During the 8-week period, different motor function tests will take niques such as immunohistochemistry and immunoblot can will and migrates up to the brain (Kim et al., 2019).

synuclein expression and dopaminergic neurons in rats with PD and without PD. Immunostaining the brain and gut tissue using an antibody that can bind selectively to alpha-synuclein will help identify primary protein of interest. Overall, this study should show the effects of a high-fat diet on motor and nonmotor symptoms in PD, and if ketogenic diet can aid PD.

Critical analysis

Further experiments should concentrate on understanding the role of diet and exercise and its effect on alpha-synuclein as a preventative measure. The study suggests that alpha-synuclein presence in the gut could become a valuable biomarker for the diagnosis of Parkinson's disease. However, a study by Recasens et al., (2018) argues against the presumptive capacity of peripheral nervous system to cause Parkinson’s disease (Recasens

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

12.

13.

14. Antony, P., Diederich, N., Krüger, R., & Balling, R. (2013). The hallmarks of Parkinson's disease. FEBS Journal, 280(23), 5981 -5993. https://doi.org/10.1111/febs.12335 Bernal-Conde, L., Ramos-Acevedo, R., Reyes-Hernández, M., Balbuena-Olvera, A., Morales-Moreno, I., & Argüero-Sánchez, R. et al. (2020). Alpha-Synuclein Physiology and Pathology: A Perspective on Cellular Structures and Organelles. Frontiers In Neuroscience, 13. https://doi.org/10.3389/fnins.2019.01399 Borghammer, P., & Hamani, C. (2017). Preventing Parkinson disease by vagotomy. Neurology, 88(21), 1982-1983. https:// doi.org/10.1212/wnl.0000000000003969 Chen, Q., Haikal, C., Li, W., & Li, J. (2019). Gut Inflammation in Association With Pathogenesis of Parkinson’s Disease. Frontiers In Molecular Neuroscience, 12. https://doi.org/10.3389/fnmol.2019.00218 de Lau, L., & Breteler, M. (2006). Epidemiology of Parkinson's disease. doi.org/10.1016/s1474-4422(06)70471-9 The Lancet Neurology, 5(6), 525-535. https://

Dickson, D. (2012). Parkinson's Disease and Parkinsonism: Neuropathology. Cold Spring Harbor Perspectives In Medicine, 2 (8), a009258-a009258. https://doi.org/10.1101/cshperspect.a009258 Fitzgerald, E., Murphy, S., & Martinson, H. (2019). Alpha-Synuclein Pathology and the Role of the Microbiota in Parkinson’s Disease. Frontiers In Neuroscience, 13. https://doi.org/10.3389/fnins.2019.00369 Hu, Q., & Wang, G. (2016). Mitochondrial dysfunction in Parkinson’s disease. https://doi.org/10.1186/s40035-016-0060-6 Translational Neurodegeneration, 5(1).

Kalia, L., & Lang, A. (2015). Parkinson's disease. The Lancet, 386(9996), 896-912. https://doi.org/10.1016/s0140-6736(14) 61393-3 Kim, S., Kwon, S., Kam, T., Panicker, N., Karuppagounder, S., & Lee, S. et al. (2019). Transneuronal Propagation of Pathologic α-Synuclein from the Gut to the Brain Models Parkinson’s Disease. Neuron, 103(4), 627-641.e7. https:// doi.org/10.1016/j.neuron.2019.05.035 Kishimoto, Y., Zhu, W., Hosoda, W., Sen, J., & Mattson, M. (2019). Chronic Mild Gut Inflammation Accelerates Brain Neuropathology and Motor Dysfunction in α-Synuclein Mutant Mice. Neuromolecular Medicine, 21(3), 239-249. https:// doi.org/10.1007/s12017-019-08539-5 Pickrell, A., Huang, C., Kennedy, S., Ordureau, A., Sideris, D., & Hoekstra, J. et al. (2015). Endogenous Parkin Preserves Dopaminergic Substantia Nigral Neurons following Mitochondrial DNA Mutagenic Stress. Neuron, 87(2), 371-381. https:// doi.org/10.1016/j.neuron.2015.06.034 Recasens, A., Carballo-Carbajal, I., Parent, A., Bové, J., Gelpi, E., Tolosa, E., & Vila, M. (2018). Lack of pathogenic potential of peripheral α-synuclein aggregates from Parkinson’s disease patients. Acta Neuropathologica Communications, 6(1). https://doi.org/10.1186/s40478-018-0509-1 Xu, L., & Pu, J. (2016). Alpha-Synuclein in Parkinson’s Disease: From Pathogenetic Dysfunction to Potential Clinical Application. Parkinson's Disease, 2016, 1-10. https://doi.org/10.1155/2016/1720621