Role of the Gut Bacteria-Derived Metabolite Phenylacetylglutamine in Health and Diseases.

Over the past few decades, it has been well established that gut microbiota-derived metabolites can disrupt gut function, thus resulting in an array of diseases. Notably, phenylacetylglutamine (PAGln), a bacterial derived metabolite, has recently gained attention due to its role in the initiation and progression of cardiovascular and cerebrovascular diseases. This meta-organismal metabolite PAGln is a byproduct of amino acid acetylation of its precursor phenylacetic acid (PAA) from a range of dietary sources like egg, meat, dairy products, etc. The microbiota-dependent metabolism of phenylalanine produces PAA, which is a crucial intermediate that is catalyzed by diverse microbial catalytic pathways. PAA conjugates with glutamine and glycine in the liver and kidney to predominantly form phenylacetylglutamine in humans and phenylacetylglycine in rodents. PAGln is associated with thrombosis as it enhances platelet activation mediated through the GPCRs receptors α2A, α2B, and ß2 ADRs, thereby aggravating the pathological conditions. Clinical evidence suggests that elevated levels of PAGln are associated with pathology of cardiovascular, cerebrovascular, and neurological diseases. This Review further consolidates the microbial/biochemical synthesis of PAGln and discusses its role in the above pathophysiologies.

Role of Endogenous Lipopolysaccharides in Neurological Disorders.

Lipopolysaccharide (LPS) is a cell-wall immunostimulatory endotoxin component of Gram-negative bacteria. A growing body of evidence reveals that alterations in the bacterial composition of the intestinal microbiota (gut dysbiosis) disrupt host immune homeostasis and the intestinal barrier function. Microbial dysbiosis leads to a proinflammatory milieu and systemic endotoxemia, which contribute to the development of neurodegenerative diseases and metabolic disorders. Two important pathophysiological hallmarks of neurodegenerative diseases (NDDs) are oxidative/nitrative stress and inflammation, which can be initiated by elevated intestinal permeability, with increased abundance of pathobionts. These changes lead to excessive release of LPS and other bacterial products into blood, which in turn induce chronic systemic inflammation, which damages the blood-brain barrier (BBB). An impaired BBB allows the translocation of potentially harmful bacterial products, including LPS, and activated neutrophils/leucocytes into the brain, which results in neuroinflammation and apoptosis. Chronic neuroinflammation causes neuronal damage and synaptic loss, leading to memory impairment. LPS-induced inflammation causes inappropriate activation of microglia, astrocytes, and dendritic cells. Consequently, these alterations negatively affect mitochondrial function and lead to increases in oxidative/nitrative stress and neuronal senescence. These cellular changes in the brain give rise to specific clinical symptoms, such as impairment of locomotor function, muscle weakness, paralysis, learning deficits, and dementia. This review summarizes the contributing role of LPS in the development of neuroinflammation and neuronal cell death in various neurodegenerative diseases.

The Role of a Gut Microbial-Derived Metabolite, Trimethylamine N-Oxide (TMAO), in Neurological Disorders.

Trimethylamine lyases are expressed in a wide range of intestinal microbiota which metabolize dietary nutrients like choline, betaine, and L-carnitine to form trimethylamine (TMA). Trimethylamine N-oxide (TMAO) is an oxidative product of trimethylamine (TMA) catalyzed by the action of flavin monooxygenases (FMO) in the liver. Higher levels of TMAO in the plasma and cerebrospinal fluid (CSF) have been shown to contribute to the development of risk factors and actively promote the pathogenesis of metabolic, cardiovascular, and cerebrovascular diseases. The investigations on the harmful effects of TMAO in the development and progression of neurodegenerative and sleep disorders are summarized in this manuscript. Clinical investigations on the role of TMAO in predicting risk factors and prognostic factors in patients with neurological disorders are also summarized. It is observed that the mechanisms underlying TMAO-mediated pathogenesis include activation of inflammatory signaling pathways such as nuclear factor kappa B (NF-κß), NOD-, LRR-, and pyrin domain-containing protein 3 (NLRP3) inflammasome, and MAPK/JNK in the periphery and brain. Data suggests that TMAO levels increase with age-related cognitive dysfunction and also induce mitochondrial dysfunction, oxidative stress, neuronal senescence, and synaptic damage in the brain. Further research into the relationships between dietary food consumption and gut microbiota-dependent TMAO levels could provide novel therapeutic options for neurological illnesses.