Beta-glucosyl-sitosterol triggers intestinal inflammation in zebrafish and mouse models possibly contributing to neurodegeneration through the gut-brain axis

Glucosylsterols (GlcSt) are lipids composed of a sterol-derived molecule and a glucose moiety, but little is known about their functions in metabolism. The alteration of their homeostasis, however, has been recognized as a risk factor for the onset of neurodegenerative diseases such as Parkinson’s Disease (PD) and Amyotrophic Lateral Sclerosis (ALS), suggesting they play a role in the physiology of the nervous system. These molecules can be synthetized endogenously, as in the case of b-glucosyl-cholesterol, or they can derive from pathogens, like -glucosyl-cholesterol. The plant-derived b-glucosyl-sitosterol (b-GlcSito), abundant in the seeds of Cycas micronesica, gained scientific relevance since it was recognized as a contributor to the so-called Amyotrophic Lateral Sclerosis-Parkinsonism Dementia Complex (ALS-PDC). This neurodegenerative disorder interested Chamorro people of Guam, who ate Cycas seeds-derived products. Among the main neurodegenerative diseases, PD is characterized by the loss of dopaminergic neurons in substantia nigra. Neuronal death is mainly associated with alteration of cell proteostasis, accumulation of proteinaceous aggregates that become cytotoxic and chronic neuroinflammatory state. Dopamine depletion is responsible for classic motor symptoms, but there is also a plethora of non-motor symptoms characteristic of an early prodromal phase or that may occur at later stages of the disease. Constipation has been recognized as a precocious hallmark that affects patients years before a possible diagnosis. ALS is a neurodegenerative disease caused by the degeneration of upper motor neurons and lower motor neurons, which innervate skeletal muscle. The key-feature of this pathology is progressive muscular wasting towards paralysis, that leads to death for respiratory failure. The presence of non-motor symptoms is peculiar also in this disorder, in which gastrointestinal defects account for constipation and delayed food transit along the digestive tract. ALS-PDC shows PD features, such as neuronal loss, proteinaceous inclusions, motor symptoms and dementia; ALS features, like muscle atrophy, or both. These observations strengthened the hypothesis that impaired GlcSt homeostasis could drive neurodegeneration, underlining the urgency of studying the impact of these molecules on central nervous system metabolism. My PhD project is focused on the characterization of the effects elicited by the plant-derived b-GlcSito using a completely new zebrafish model treated with this molecule and, in parallel, to deepen the knowledge on the actions triggered in the mouse model. In the first chapter, I present the results obtained exploiting our zebrafish model, at larval and adult life stages. Direct b-GlcSito absorption through the water or following prolonged treatment with a b-GlcSito-enriched-diet seems to cause a gut inflammatory state characterized by altered gene expression and motility. Intestinal dysfunction is paralleled by the impairment of pathways associated with the occurrence of neurodegeneration, like autophagy. In the second chapter, I describe the data obtained from the mouse model fed with b-GlcSito-enriched diet. The increase in the levels of this molecule seems to induce changes in the morphology of gut epithelium, besides the alteration of genes involved in the modulation of inflammation, as revealed by RNAseq of intestinal tissue. Evidence of incoming neurodegeneration was also established through gene and protein expression analyses. In the third chapter, I present preliminary results on b-GlcSito possible mechanism of action. Due to its molecular structure it could interact with steroid hormone receptors, such as the Glucocorticoid Receptor (GR). Thanks to zebrafish mutant lines devoid of genes involved in glucocorticoid synthesis and metabolism, we tried to assess whether b-GlcSito could interact with GR and influence the expression of GR-related genes. Our data seem to support this hypothesis.