Homostachydrine is a Xenobiotic Substrate of OCTN1/SLC22A4 and Potentially Sensitizes Pentylenetetrazole-Induced Seizures in Mice

ArcKR expression modifies synaptic plasticity following epileptic activity: Differential effects with in vitro and in vivo seizure-induction protocols

OBJECTIVES

Pathological forms of neural activity, such as epileptic seizures, modify the expression pattern of multiple proteins, leading to persistent changes in brain function. One such protein is activity-regulated cytoskeleton-associated protein (Arc), which is critically involved in protein-synthesis-dependent synaptic plasticity underlying learning and memory. In the present study, we have investigated how the expression of ArcKR, a form of Arc in which the ubiquitination sites have been mutated, resulting in slowed Arc degradation, modifies group I metabotropic glutamate receptor-mediated long-term depression (G1-mGluR-LTD) following seizures.

METHODS

We used a knock-in mice line that express ArcKR and two hyperexcitation models: an in vitro model, where hippocampal slices were exposed to zero Mg2+, 6 mM K+; and an in vivo model, where kainic acid was injected unilaterally into the hippocampus. In both models, field excitatory postsynaptic potentials (fEPSPs) were recorded from the CA1 region of hippocampal slices in response to Schaffer collateral stimulation and G1-mGluR-LTD was induced chemically with the group 1 mGluR agonist DHPG.

RESULTS

In the in vitro model, ArcKR expression enhanced the effects of seizure activity and increased the magnitude of G1-mGluR LTD, an effect that could be blocked with the mGluR5 antagonist MTEP. In the in vivo model, fEPSPs were significantly smaller in slices from ArcKR mice and were less contaminated by population spikes. In this model, the amount of G1-mGluR-LTD was significantly less in epileptic slices from ArcKR mice as compared to wildtype (WT) mice.

SIGNIFICANCE

We have shown that expression of ArcKR, a form of Arc in which degradation is reduced, significantly modulates the magnitude of G1-mGluR-LTD following epileptic seizures. However, the effect of ArcKR on LTD depends on the epileptic model used, with enhancement of LTD in an in vitro model and a reduction in the kainate mouse model.

Inflammation and Organic Cation Transporters Novel (OCTNs)

Inflammation is a physiological condition characterized by a complex interplay between different cells handled by metabolites and specific inflammatory-related molecules. In some pathological situations, inflammation persists underlying and worsening the pathological state. Over the years, two membrane transporters namely OCTN1 (SLC22A4) and OCTN2 (SLC22A5) have been shown to play specific roles in inflammation. These transporters form the OCTN subfamily within the larger SLC22 family. The link between these proteins and inflammation has been proposed based on their link to some chronic inflammatory diseases such as asthma, Crohn's disease (CD), and rheumatoid arthritis (RA). Moreover, the two transporters show the ability to mediate the transport of several compounds including carnitine, carnitine derivatives, acetylcholine, ergothioneine, and gut microbiota by-products, which have been specifically associated with inflammation for their anti- or proinflammatory action. Therefore, the absorption and distribution of these molecules rely on the presence of OCTN1 and OCTN2, whose expression is modulated by inflammatory cytokines and transcription factors typically activated by inflammation. In the present review, we wish to provide a state of the art on OCTN1 and OCTN2 transport function and regulation in relationships with inflammation and inflammatory diseases focusing on the metabolic signature collected in different body districts and gene polymorphisms related to inflammatory diseases.

Ergothioneine and central nervous system diseases

Ergothioneine (ERGO) is a thiol contained in the food that exhibits an excellent antioxidant effect similar to that of glutathione. Although mammals lack a biosynthetic pathway for ERGO, the carnitine/organic cation transporter OCTN1/SLC22A4, which transports ERGO in vivo, is expressed throughout the body, and ERGO is distributed to various organs after oral intake. ERGO is a stable compound that remains in the body for a long time after ingestion. OCTN1 is also expressed in brain parenchymal cells, including neurons, and ERGO in the blood permeates the blood-brain barrier and is distributed to the brain, exhibiting a neuroprotective effect. Recently, the association between central nervous system (CNS) diseases and ERGO has become a research focus. ERGO concentrations in the blood components are lower in patients with cognitive impairment, Parkinson's disease, and frailty than in healthy subjects. ERGO exerts a protective effect against various neurotoxins and improves the symptoms of cognitive impairment, depression, and epilepsy in animal models. The promotion of neurogenesis and induction of neurotrophic factors, in addition to the antioxidant and anti-inflammatory effects, may be involved in the neuroprotective effect of ERGO. This review shows the association between ERGO and CNS diseases, discusses the possible biomarkers of peripheral ERGO in CNS diseases, and the possible preventive and improvement effects of ERGO on CNS diseases.

OCTN1: A Widely Studied but Still Enigmatic Organic Cation Transporter Linked to Human Pathology and Drug Interactions

The Novel Organic Cation Transporter, OCTN1, is the first member of the OCTN subfamily; it belongs to the wider Solute Carrier family SLC22, which counts many members including cation and anion organic transporters. The tertiary structure has not been resolved for any cation organic transporter. The functional role of OCNT1 is still not well assessed despite the many functional studies so far conducted. The lack of a definitive identification of OCTN1 function can be attributed to the different experimental systems and methodologies adopted for studying each of the proposed ligands. Apart from the contradictory data, the international scientific community agrees on a role of OCTN1 in protecting cells and tissues from oxidative and/or inflammatory damage. Moreover, the involvement of this transporter in drug interactions and delivery has been well clarified, even though the exact profile of the transported/interacting molecules is still somehow confusing. Therefore, OCTN1 continues to be a hot topic in terms of its functional role and structure. This review focuses on the most recent advances on OCTN1 in terms of functional aspects, physiological roles, substrate specificity, drug interactions, tissue expression, and relationships with pathology.

Ergothioneine in the brain

Ergothioneine (ERGO) is a naturally occurring food-derived antioxidant. Despite its extremely hydrophilic properties, ERGO is easily absorbed from the gastrointestinal tract and distributed to various organs, including the brain. This is primarily because its entry into brain cells is mediated by the ERGO-specific transporter OCTN1/SLC22A4. Octn1 gene knockout mice do not have ERGO in the brain, due to the absence of OCTN1 in neurons, neural stem cells, and microglia. The existence of OCTN1 and uptake of ERGO into the brain parenchymal cells may suggest that ERGO and its transporter play a pivotal role in brain function. Oral administration of ERGO has antidepressant activities in mice. Furthermore, repeated oral administration of ERGO and ERGO-containing food extract tablets enhance memory function in mice and humans, respectively. ERGO also protects against stress-induced sleep disturbance and neuronal injury induced by amyloid β in rodents. In vitro observations suggest that ERGO benefits brain function through both its antioxidative activity and by promoting neurogenesis and neuronal maturation. This review discusses the possible involvement of ERGO in brain function and its potential therapeutic properties.

The Ergothioneine Transporter (ETT): Substrates and Locations, an Inventory

In all vertebrates including mammals, the ergothioneine transporter ETT (obsolete name OCTN1; human gene symbol SLC22A4) is a powerful and highly specific transporter for the uptake of ergothioneine (ET). ETT is not expressed ubiquitously and only cells with high ETT cell-surface levels can accumulate ET to high concentration. Without ETT, there is no uptake because the plasma membrane is essentially impermeable to this hydrophilic zwitterion. Here, we review the substrate specificity and localization of ETT, which is prominently expressed in neutrophils, monocytes/macrophages, and developing erythrocytes. Most sites of strong expression are conserved across species, but there are also major differences. In particular, we critically analyze the evidence for the expression of ETT in the brain as well as recent data suggesting that the transporter SLC22A15 may transport also ET. We conclude that, to date, ETT remains the only well-defined biomarker for intracellular ET activity. In humans, the ability to take up, distribute, and retain ET depends principally on this transporter.

Ergothioneine, recent developments

There has been a recent surge of interest in the unique low molecular weight dietary thiol/thione, ergothioneine. This compound can accumulate at high levels in the body from diet and may play important physiological roles in human health and development, and possibly in prevention and treatment of disease. Blood levels of ergothioneine decline with age and onset of various diseases. Here we highlight recent advances in our knowledge of ergothioneine.