4-Ethylphenol-fluxes, metabolism and excretion of a gut microbiome derived neuromodulator implicated in autism

Abstract

Gut-microbiome-derived metabolites, such as 4-Ethylphenol [4EP], have been shown to modulate neurological health and function. Although the source of such metabolites is becoming better understood, knowledge gaps remain as to the mechanisms by which they enter host circulation, how they are transported in the body, how they are metabolised and excreted, and the way they exert their effects. High blood concentrations of host-modified 4EP, 4-ethylphenol sulfate [4EPS], are associated with an anxiety phenotype in autistic individuals. We have reviewed the existing literature and discuss mechanisms that are proposed to contribute influx from the gut microbiome, metabolism, and excretion of 4EP. We note that increased intestinal permeability is common in autistic individuals, potentially explaining increased flux of 4EP and/or 4EPS across the gut epithelium and the Blood Brain Barrier [BBB]. Similarly, kidney dysfunction, another complication observed in autistic individuals, impacts clearance of 4EP and its derivatives from circulation. Evidence indicates that accumulation of 4EPS in the brain of mice affects connectivity between subregions, particularly those linked to anxiety. However, we found no data on the presence or quantity of 4EP and/or 4EPS in human brains, irrespective of neurological status, likely due to challenges sampling this organ. We argue that the penetrative ability of 4EP is dependent on its form at the BBB and its physicochemical similarity to endogenous metabolites with dedicated active transport mechanisms across the BBB. We conclude that future research should focus on physical (e.g., ingestion of sorbents) or metabolic mechanisms (e.g., conversion to 4EP-glucuronide) that are capable of being used as interventions to reduce the flux of 4EP from the gut into the body, increase the efflux of 4EP and/or 4EPS from the brain, or increase excretion from the kidneys as a means of addressing the neurological impacts of 4EP.

Keywords: 4-ethylphenol; 4-ethylphenol sulfate; autism; gut microbiome; host microbiome modulation; membrane integrity; neuromodulators; uremic toxins.

FIGURE 1

Three pathways may contribute to 4EP formation in the colon. Pathways 1 and 2 converge (coloured blue) and the enzyme names required to catalyse specific reactions are annotated above the arrow. Except for the final reaction (annotated *Decarboxylase*), the pathway in which 4EP may be made in a way similar to p-cresol (pathway 3, green) is well-established (32). Therefore, enzyme and metabolite names have been omitted. Following formation in the colon 4EP crosses the epithelial membrane to the hepatic portal vein via an unknown mechanism. When 4EP enters the liver it is sulphated by sulfotransferase SULT1 to form 4EPS (33). 4EPS subsequently leaves the liver and travels to peripheral tissues through the circulatory system. PAL, Phenylalanine ammonia-lyase; C4H, Cinnamate 4-hydroxylase; PHL, phenylalanine hydroxylase; THL, tyrosine hydroxylase; DHH, decarboxylate hydroxycinnamic acid hydroxylase; VPR, Vinyl phenol reductase (3, 26–28).

FIGURE 2

4EP can move through the colon epithelial membrane by three possible mechanisms. (A) Active transport of 4EP and/or 4EPS through the membrane. (B) Localised mis formation of tight junctions forms paracellular gaps in the membrane and allows 4EP and/or 4EPS to leak through the colon epithelial membrane. (C) 4EP passively diffuses through the lipid bilayer due to its small size, relative non-polarity, and lack of charge. 4EPS does not pass through the phospholipid bilayer due to its charged and polar metabolite characteristics.

FIGURE 3

Possible transport mechanisms of 4EP and negatively charged 4EPS, through the BBB. (A) 4EPS is actively transported through the apical and basolateral membrane. (B) 4EP passively diffuses through the lipid bilayer due to its small size, solubility and lack of charge. (C) 4EPS is transported into the BBB epithelial cell through an influx transporter. Once in the cell, 4EPS is desulfated to 4EP, which can then passively diffuse into the brain. (D) Delocalised tight junction formation results in paracellular gaps in the membrane that allow 4EP and/or 4EPS to pass through the barrier. (E) Efflux transporters actively transport 4EPS out of the brain, e.g., OATs. BBB, blood brain barrier.

FIGURE 4

(A) 4EP (B) pregnenolone sulphate. Red squares show where the STS enzyme binds to pregnenolone sulphate and where we suggest an STS enzyme may bind to 4EPS to desulfate to 4EP.

FIGURE 5

(A) Catechol-o-sulfate (B) 4EPS.